rust: kernel: add `fmt` module

rust: kernel: add `fmt` module

`kernel::fmt` is a facade over `core::fmt` that can be used downstream,
allowing future changes to the formatting machinery to be contained
within the kernel crate without downstream code needing to be modified.

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-v1-2-a91524037783@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>