Interpreter: ext annotation method now takes an empty collection to know the type

In case there's an empty collection as input. In future, we can get this
from the typechecker.
Also, added type information to type mismatch error in collection.
Added coerce builtin to convert collection types

This is needed to convert between, say, a set and a seq. We do this in
the old k3 in a few places, and there's no easy way to bypass that.
New rst.k3