Enhancement: Add a formatting method to detect truncation

[tttwang23]

/// Format using a stack buffer. Result in an error if there is truncation.
/// A reference to str is returned in Result.
pub fn fmt_stack<'a>(buffer: &'a mut [u8], args: fmt::Arguments) -> Result<&'a str, fmt::Error> {
let mut w = stackfmt::WriteTo::new(buffer);
fmt::write(&mut w, args)?;
if w.overflow {
Err(fmt::Error)
}
else {
Ok(w.as_str())
}
}

[tttwang23]

Also would like to see the 'overflow' flag to have a public method as 'w.has_overflow()'.