For some reasons, I cannot compile correctly this library with inline
functions using Meson and Ninja (commit adding Meson build file
incoming).
Some attributes are also added, such as an attribute for all functions
receiving pointers as arguments telling these arguments cannot be null.
Functions that end up calling a `malloc' or `realloc' return the new
struct `Result' which indicates if the memory allocation or reallocation
worked or not. If not, it holds a message, otherwise it holds the value
the function is meant to return (can be a null pointer if the function
returned a `void' before). The library will no longer forcibly exit the
program on such error in case it can be recovered by the code calling
it.
The modifications described above lead to vector-creating functions
returning a pointer to the newly created vector, and the function for
deleting vectors also frees the vector.
This commit also inlines some functions to make the compiled code a bit
lighter and calls to these functions possibly a bit faster.
`PDEB' is redefined following the new `PCOMMON' macro that is also used
by `PERR' for desplaying debug error messages.
Some lines are also modified to avoid potential memory leaks in case of
a memory (re)allocation.
