rust: str: add `CStr` methods matching `core::ffi::CStr`

rust: str: add `CStr` methods matching `core::ffi::CStr`

Prepare for replacing `CStr` with `core::ffi::CStr` by soft-deprecating
methods which don't exist on `core::ffi::CStr`.

We could keep `as_bytes{,_with_nul}` through an extension trait but
seeing as we have to introduce `as_char_ptr_in_const_context` as a free
function, we may as well introduce `to_bytes{,_with_nul}` here to allow
downstream code to migrate in one cycle rather than two.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250704-core-cstr-prepare-v1-5-a91524037783@gmail.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>