Clean up most warnings in Interpreter.

The only ones left are those pertaining to "declared but not used" -- must evaluate.
Fix missing type-signatures in L.K3.Pretty.
Fix warnings in L.K3.C.Type.
Fix warnings in L.K3.C.Expression.
Fix warnings in L.K3.C.Declaration.
Fix warnings in L.K3.C.C.{Type,Declaration}.