Use the resolved type name in generic strings, not the original

This makes some generics less readable, but some more, and reduces
generic type pollution. Some cases are a loss, eg
`Option_PaymentHashZ` gets converted to `Option_ThirtyTwoBytesZ`
(which less less readable but fewer total types as a result), but
we ultimately need this because we now have `schnorr::Signature`
and `ecdsa::Signature` so we can't rely on just using `Signature`
and having it be unique :(.