Create a `Default` trait for an empty format string

See discussion in #8489, but this selects option 3 by adding a `Default` trait to be implemented by various basic types.

Once this makes it into a snapshot I think it's about time to start overhauling all current use-cases of `fmt!` to move towards `ifmt!`. The goal is to replace `%X` with `{}` in 90% of situations, and this commit should enable that.