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.
@brson grilled me about how this bugfix worked the first time around, and it occurred to me that it didn't in the case where the task is unwinding. Now it will.
