rust: transmute: add `as_bytes` method for `AsBytes` trait

rust: transmute: add `as_bytes` method for `AsBytes` trait

Every type that implements `AsBytes` should be able to provide its byte
representation. Introduce the `as_bytes` method that returns the
implementer as a stream of bytes, and provide a default implementation
that should be suitable for any type that satisfies `AsBytes`'s safety
requirements.

[acourbot@nvidia.com: use fully qualified `core::mem::size_of_val` to
build with Rust 1.78.]

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250801-as_bytes-v5-1-975f87d5dc85@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>