[C#] Use `Heap{Alloc,Free}` rather than `malloc`/`free` on Windows

[C#] Use `Heap{Alloc,Free}` rather than `malloc`/`free` on Windows

We generally expect to allocate pointers in C which Rust will free,
and vice versa. On Windows, this requires that we use the same
allocation path as Rust's stdlib does, which is actually
`Heap{Alloc,Free}`, not `malloc`/`free`. Here, we swap the heap
allocation functions we call, fixing heap corruption issues on
Windows.