rust: types: introduce ForeignOwnable

It was originally called `PointerWrapper`. It is used to convert
a Rust object to a pointer representation (void *) that can be
stored on the C side, used, and eventually returned to Rust.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/lib.rs

@@ -16,6 +16,7 @@
 #![feature(coerce_unsized)]
 #![feature(core_ffi_c)]
 #![feature(dispatch_from_dyn)]
+#![feature(generic_associated_types)]
 #![feature(receiver_trait)]
 #![feature(unsize)]