rust: init: add initialization macros

Add the following initializer macros:
- `#[pin_data]` to annotate structurally pinned fields of structs,
  needed for `pin_init!` and `try_pin_init!` to select the correct
  initializer of fields.
- `pin_init!` create a pin-initializer for a struct with the
  `Infallible` error type.
- `try_pin_init!` create a pin-initializer for a struct with a custom
  error type (`kernel::error::Error` is the default).
- `init!` create an in-place-initializer for a struct with the
  `Infallible` error type.
- `try_init!` create an in-place-initializer for a struct with a custom
  error type (`kernel::error::Error` is the default).

Also add their needed internal helper traits and structs.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-8-y86-dev@protonmail.com
[ Fixed three typos. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/macros/quote.rs

@@ -38,7 +38,6 @@ impl ToTokens for TokenStream {
 /// This is a similar to the
 /// [`quote_spanned!`](https://docs.rs/quote/latest/quote/macro.quote_spanned.html) macro from the
 /// `quote` crate but provides only just enough functionality needed by the current `macros` crate.
-#[allow(unused_macros)]
 macro_rules! quote_spanned {
     ($span:expr => $($tt:tt)*) => {
     #[allow(clippy::vec_init_then_push)]
@@ -137,7 +136,6 @@ macro_rules! quote_spanned {
 /// `macros` crate.
 ///
 /// [`Span::mixed_site()`]: https://doc.rust-lang.org/proc_macro/struct.Span.html#method.mixed_site
-#[allow(unused_macros)]
 macro_rules! quote {
     ($($tt:tt)*) => {
         quote_spanned!(::proc_macro::Span::mixed_site() => $($tt)*)