[Emacs] Add new Rust snippets, common-lisp layer

This commit adds two new snippets for Rust, one for an easier way to
write a `println' macro than the default one, and another one for
writing more easily new `new' functions for Rust structs.

It also adds the `common-lisp' layer in Emacs so I can test StumpWM.

.config/emacs/private/snippets/rust-mode/println

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: println!("{}", value);
+# key: pln
+# --
+println!("${1:{}}", $2);