rust: add `Alignment` type

rust: add `Alignment` type

Alignment operations are very common in the kernel. Since they are
always performed using a power-of-two value, enforcing this invariant
through a dedicated type leads to fewer bugs and can improve the
generated code.

Introduce the `Alignment` type, inspired by the nightly Rust type of the
same name and providing the same interface, and a new `Alignable` trait
allowing unsigned integers to be aligned up or down.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
[ Used `build_assert!`, added intra-doc link, `allow`ed
  `clippy::incompatible_msrv`, added `feature(const_option)`, capitalized
  safety comment. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>