Allow term embeddings for typeclass instances #12

Right now, we allow term embeddings for anything with a TH.Name. This excludes typeclass methods: their definition has a name but not their instance. We should have some syntax to permit a user to override a typeclass method. On the implementation side, it is not super trivial as the function also does not directly have a name inside of the evaluator either. We should probably instead adjust the DFunId to have contain the new method instead? I guess this doesn't propagate to other DFunIds already in the code though? Perhaps the DFunId should be part of the substitution environment and we can first lookup there before we check the unfolding. Probably depends on #4 as this changes code here quite a bit!