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>

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