rust: prelude: add `pin-init` API items to prelude

Add `pin-init` API macros and traits to the prelude.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-13-y86-dev@protonmail.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>