Remove package, update layers, remove custom mu, Rust update

The `meson-mode` package is removed from my custom package list with
the addition of the `meson` layer in Spacemacs.

The `org` layer’s support for Trello is also removed from my
configuration while the support for Hugo is added.

The `c-c++` layer now uses `clangd` as its LSP backend instead of ccls

The `haskell` layer is removed from my configuration

Since it is now broken, instructions for installing a custom `mu`
package on Arch Linux that still enables it has been removed. I will
now use the default `mu` package from the AUR.

The Rust configuration has been updated with some additional
instructions on the reason why some elements are set, and how to
install these. The Rust LSP backend will now use the command `cargo
clippy` instead of `cargo check` to catch errors and warnings.

org/config/spacemacs.org

@@ -223,7 +223,6 @@
    | kaolin-themes       | some cool themes                                     |
    | lsp-dart            | apparently, it isn’t included in the Dart layer      |
    | magit-gitflow       | integrate gitflow in Magit                           |
-   | meson-mode          | major mode for Meson build files                     |
    | multiple-cursors    | I don’t like the layer, I prefer this package alone  |
    | org-sidebar         | display on the side the outline of an Org buffer     |
    | outorg              | edit comments as Org-mode buffers                    |
@@ -371,8 +370,8 @@
        (org :variables
             org-enable-epub-support t
             org-enable-github-support t
+            org-enable-hugo-support t
             org-enable-reveal-js-support t
-           org-enable-trello-support t
             org-enable-sticky-header t
             spaceline-org-clock-p t
             org-projectile-file "TODOs.org"
@@ -534,8 +533,7 @@
               c-c++-adopt-subprojects t
               c-c++-enable-google-style t
               c-c++-enable-c++11 t
-              c-c++-backend 'lsp-ccls
+              c-c++-backend 'lsp-clangd
-              c-c++-lsp-executable "/usr/bin/ccls"
               c-c++-lsp-sem-highlight-method 'overlay
               c-c++-lsp-sem-highlight-rainbow t
               c++-enable-organize-includes-on-save t)
@@ -597,10 +595,6 @@
             lua-lsp-emmy-enable-file-watchers t)
      #+END_SRC
 
-     #+BEGIN_SRC emacs-lisp
-haskell
-     #+END_SRC
-
 *** Readers
     :PROPERTIES:
     :CUSTOM_ID: Spacemacs_layers_and_packages-Layers-Readers-65e8e4ae
@@ -658,10 +652,11 @@ haskell
     #+END_SRC
 
     Next, we have the Docker, Nginx,  Pass (the standard Unix password manager),
-    Prettier, Systemd,  Imenu-list, Web-beautify,  Dap, Helpful, and  LSP layers
+    Prettier, Systemd,  Meson, Imenu-list,  Web-beautify, Dap, Helpful,  and LSP
-    enabled.
+    layers enabled.
     #+BEGIN_SRC emacs-lisp
-      docker imenu-list nginx pass prettier systemd web-beautify helpful dap lsp
+      dap docker helpful imenu-list lsp meson nginx pass prettier systemd
+      web-beautify
     #+END_SRC
 
     We  also have  the RestClient  layer  enabled for  which I  enabled the  Org
@@ -1919,69 +1914,10 @@ haskell
    :CUSTOM_ID: User_Configuration-Mu4e-f3df8e9e
    :END:
    Mu4e is a frontend  for mu, an email analyzer which sits on  top of a Maildir
-   which gets updated with ~isync~. It has a lot of neat  features, but I guess
+   which gets updated  with the ~mbsync~ command  from ~isync~. It has  a lot of
-   my favorite ones are:
+   neat features, but I guess my favorite ones are:
    1. the search query feature
    2. being able to send an HTML email either to the browser or
-   3. to compile it to a PDF for a better output
-   This  last feature  can only  be enabled  manually with  Arch Linux.  We need
- ~webkitgtk~ installed,  and unfortunately,  it is  by default  only available
-   through the  AUR and  has to be  compiled. Thing is, ~webkitgtk~ is  a *BIG*
-   piece of software, and compiling it can be very time consuming. So, I decided
-   to add ArchLinuxCN’s  servers to my pacman config (instructions  on how to do
-   so [[https://github.com/archlinuxcn/repo][here]]) so I could install a compiled version from their repos. Next, when I
-   installed ~mu~ from the AUR, I modified the PKGBUILD like so:
-   #+BEGIN_SRC text :tangle no
-     # Maintainer: Marcel Röthke <marcel@roethke.info>
-     # Contributor: Pierre Neidhardt <ambrevar@gmail.com>
-     # Contributor: csllbr; Popsch <popsch@gmx.net>
-
-     pkgname=mu
-     pkgver=1.2
-     pkgrel=3
-     pkgdesc="Maildir indexer/searcher and Emacs client (mu4e)"
-     arch=("x86_64")
-     url="http://www.djcbsoftware.nl/code/mu"
-     license=("GPL")
-     depends=("gmime3" "xapian-core" "guile2.0")
-     makedepends=("emacs")
-     optdepends=("guile: guile support"
-       "emacs: mu4e support")
-     source=("mu-$pkgver.tar.gz::https://github.com/djcb/mu/archive/$pkgver.tar.gz")
-     md5sums=('5e7d7c3549b9428ec0b5ea7d374b6d0d')
-
-     prepare() {
-       cd "$pkgname-$pkgver"
-       autoreconf -i
-     }
-
-
-     build() {
-       cd "$pkgname-$pkgver"
-       ./configure --prefix=/usr --enable-mu4e --enable-guile
-       make
-     }
-
-     package() {
-       cd "$pkgname-$pkgver"
-       make DESTDIR="$pkgdir" install
-       # if msg2pdf and mug were built, install them
-       # if not, remove the unneeded mug manpage
-       if [ -f toys/msg2pdf/msg2pdf ]; then
-         install -m755 toys/msg2pdf/msg2pdf "${pkgdir}"/usr/bin/msg2pdf
-       fi
-       if [ -f toys/mug/mug ]; then
-         install -m755 toys/mug/mug "${pkgdir}"/usr/bin/mug
-         install -Dm644 toys/mug/mug.svg "${pkgdir}"/usr/share/pixmaps/mug.svg
-       else
-         rm "${pkgdir}"/usr/share/man/man1/mug.1
-       fi
-     }
-   #+END_SRC
-
-   This enables the  installation of ~msg2pdf~, which in turn  enables the email
-   compilation to PDF with the shortcut ~a  v~ when an email is opened, and this
-   opens the PDF in a new Emacs buffer.
 
    Due to mu sitting on top of a Maildir, I need to tell mu4e where said maildir
    is, and point it the trash, archive,  and sent folders as well as the refresh
@@ -3069,9 +3005,31 @@ haskell
    :END:
    I need to  point to racer where the  source code of Rust is located  so I can
    get some documentation.  This is installed with the ~rust-src~ component you
-   can get through ~rustup~.
+   can get through ~rustup~. To install it, simply run
+   #+BEGIN_SRC shell :tangle no :exports code
+     $ rustup component add rust-src
+   #+END_SRC
+
+   Now, the  source code  for Rust  should be included  in your  installation. I
+   personally prefer to develop with Rust  stable, so let’s indicate to Emacs to
+   search for documentation in the stable sources:
    #+BEGIN_SRC emacs-lisp
-     (setq racer-rust-src-path "~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src")
+     (setq racer-rust-src-path
+           "~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src")
+   #+END_SRC
+
+   Rust’s default ~cargo check~ command  is already  very good, however  I also
+   enjoy getting  some more hints  while developping,  and ~clippy~ does  a very
+   good job at it. To get clippy, I need to run the following to install it:
+   #+BEGIN_SRC shell
+     $ rustup compontent add clippy
+   #+END_SRC
+
+   And this will get it installed with all  of my Rust toolchain, and it will be
+   updated with it. Now,  let’s indicate LSP that I want to  use that instead of
+ ~check~:
+   #+BEGIN_SRC emacs-lisp
+     (setq lsp-rust-analyzer-cargo-watch-command "clippy")
    #+END_SRC
 
    Finally, I  wish to  enable ~electric-pair-mode~ and ~indent-guide-mode~ for