rust: str: add `Formatter` type

Add the `Formatter` type, which leverages `RawFormatter`,
but fails if callers attempt to write more than will fit
in the buffer.

In order to so, implement the `RawFormatter::from_buffer()`
constructor as well.

Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
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>