Make MIN_ALIGN a const to allow better optimization

With MIN_ALIGN as a static, other crates don't have access to its value
at compile time, because it is an extern global. That means that the
checks against it can't be optimized out, which is rather unfortunate.
So let's make it a constant instead.
rm unnecessary libc allocator usage
get rid of libc_heap::{malloc_raw, realloc_raw}

The C standard library functions should be used directly. The quirky
NULL / zero-size allocation workaround is no longer necessary and was
adding an extra branch to the allocator code path in a build without
jemalloc. This is a small step towards liballoc being compatible with
handling OOM errors instead of aborting (#18292).

[breaking-change]
return the new usable size from reallocate_inplace

The real size is also more useful than just a boolean, and the caller
can easily determine if the operation failed from the real size. In most
cases, the caller is only going to be growing the allocation so a branch
can be avoided.

[breaking-change]
fix sized deallocation documentation
auto merge of #18293 : thestinger/rust/heap, r=cmr