rust: bits: add support for bits/genmask macros

rust: bits: add support for bits/genmask macros

In light of bindgen being unable to generate bindings for macros, and
owing to the widespread use of these macros in drivers, manually define
the bit and genmask C macros in Rust.

The *_checked version of the functions provide runtime checking while
the const version performs compile-time assertions on the arguments via
the build_assert!() macro.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250714-topics-tyr-genmask2-v9-1-9e6422cbadb6@collabora.com
[ `expect`ed Clippy warning in doctests, hid single `use`, grouped
  examples. Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>