rust: pin-init: Add the `Wrapper` trait.

This trait allows creating `PinInitializers` for wrapper or new-type
structs with the inner value structurally pinned, when given the
initializer for the inner value.

Implement this trait for `UnsafeCell` and `MaybeUninit`.

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://github.com/Rust-for-Linux/pin-init/pull/37/commits/3ab4db083bd7b41a1bc23d937224f975d7400e50


[ Reworded commit message into imperative mode, fixed typo and fixed
  commit authorship. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>