Reinstate specialization of Perm on the vector argument type.

This fixes the previous commit's performance regression for sortperm.

Also note that if you don't import Sort.Reverse into Sort.Float, then
reverse sorting performance tanks and the type-inferred, lowered code
for forward and reverse sorting end up rather alarmingly different.
This happens because there is a type in Base called Reverse and the
inference seems to get confused. There should definitely be some kind
of warning if such confusion occurs.