Adjust the handling of trait obligations and defaults to account for

Adjust the handling of trait obligations and defaults to account for
upvar inference.  Upvar inference can cause some obligations to be
deferred, notably things like `F : Sized` where `F` is a closure type,
or `F : FnMut`. Adjust the ordering therefore so that we process all
traits and apply fallback, do upvar inference, and only then start
reporting errors for outstanding obligations.