This removes the stacking of type parameters that occurs when invoking
trait methods, and fixes all places in the standard library that were
relying on it. It is somewhat awkward in places; I think we'll probably
want something like the `Foo::<for T>::new()` syntax.
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.
