rust: init: add `assert_pinned` macro

Add a macro to statically check if a field of a struct is marked with
`#[pin]` ie that it is structurally pinned. This can be used when
`unsafe` code needs to rely on fields being structurally pinned.

The macro has a special "inline" mode for the case where the type
depends on generic parameters from the surrounding scope.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240814-linked-list-v5-1-f5f5e8075da0@google.com


[ Replaced `compile_fail` with `ignore` and a TODO note. Removed
  `pub` from example to clean `unreachable_pub` lint. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>