Remove `const` qualifier from the argument of `free()` and `realloc()`,

Remove `const` qualifier from the argument of `free()` and `realloc()`,
as well as in numerous other variables that hold ownership of memory.

By convention, a pointer that holds ownership is _never_ qualified by `const`.
This is reflected in the standard type signature of `free()` and `realloc()`.
Allowing const pointers to hold ownership may seem superficially convenient,
but is actually quite confusing to experienced C programmers.