Add a `fmt::Display` wrapper type

[rotty]

By adding a type similar to the DisplayFileSize struct sketched below, it would be possible to:

• Give a more convenient alternative API.
• Avoid a heap allocation when formatting.

The end-user API could look like this:

println!("file size: {}", humansize::display(1000, humansize::BINARY));

Below is an implementation I added to my code based on the current FileSize trait, which obviously doesn't take advantage of the opportunity to avoid allocation.

@LeopoldArkham: If you think that this is a good idea, I'd gladly try submit a PR implementing this API extension, which should be doable in a backwards-compatible fashion.

struct DisplayFileSize<'a, T> {
    size: T,
    opts: &'a humansize::file_size_opts::FileSizeOpts,
}

impl<'a, T> fmt::Display for DisplayFileSize<'a, T>
where
    T: fmt::Display + humansize::FileSize,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let s = self.size.file_size(self.opts).map_err(|_| fmt::Error::default())?;
        write!(f, "{}", s)
    }
}

fn display_file_size<'a, T>(
    size: T,
    opts: &'a humansize::file_size_opts::FileSizeOpts,
) -> DisplayFileSize<'a, T> {
    DisplayFileSize { size, opts }
}