Update internal usage of mirrors to use refinement types

[mpilquist]

Now that scala/scala3#7380 has been fixed, we can replace this idiom:

using m: Mirror.ProductOf[P], i: m.MirroredElemTypes =:= A *: EmptyTuple

with:

using m: Mirror.ProductOf[P] { type MirroredElemTypes = A *: EmptyTuple }

To retain binary compatibility, we'd have to retain the existing signatures as private[skunk] for the various operations that have already been released. See scodec/scodec@919f964 for a similar change that was done to scodec.