FormatWith should implement fmt::Debug

[m-mueller678]

The value returned by format_with should implement std::fmt::Debug. Similar to the value returned by std::format_args, the Debug representation should be the same as the Display one. This allows using it in places like std::fmt::DebugStruct.

[Philippe-Cholet]

Similarly to https://doc.rust-lang.org/src/core/fmt/mod.rs.html#451 we could indeed delegate to Display:

impl<'a, I, F> fmt::Debug for FormatWith<'a, I, F>
where
    I: Iterator,
    F: FnMut(I::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(self, f)
    }
}

I don't see an issue with this, but I'm not familiar with those Format/FormatWith.
@phimuemue @jswrenn Any objection?