Note [Using TcM for zonking to Type]

The main zonking monads currently wrap TcM, because we need access to
the full TcM monad in order to expand typed TH splices.
See zonkExpr (HsTypedSplice s _) = ...

After the Typed TH plan has been implemented, this should no longer be necessary,
and we should be able to use a stripped down monad, similar to the ZonkM monad
which we use for zonking within the typechecker (but we will need a place to
accumulate errors).