rust: str: add `fmt!` macro

Add the `fmt!` macro, which is a convenience alias for the Rust
`core::format_args!` macro.

For instance, it may be used to create a `CString`:

    CString::try_from_fmt(fmt!("{}{}", "abc", 42))?

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>