Makes FFI-MEM-WRAPPING example a bit more Rusty

[Voronar]

No description provided.

[danielhenrymantilla]

I personally do like the changes; I'd kind of suggest that ptr::NonNull<Foo> be used instead of a through-&mut raw pointer, because the semantics w.r.t. aliasing are not exactly the same, and that when aliasing is involved things would be a bit more subtle. Raw pointers are better at dodging aliasing assumptions with the compiler:

- pub struct Foo(*mut RawFoo);
+ pub struct Foo(ptr::NonNull<RawFoo>);

  …

  impl Foo {
      /// Create a Foo
      pub fn new() -> Option<Foo> {
          // The `?` operator here ensures `None` is returned on `NULL`.
-         let raw_ptr = unsafe { foo_create().as_mut()? };
+         let raw_ptr = ptr::NonNull::new(unsafe { foo_create() })?;

etc.