Builds a Codec[M] using the supplied function M => A.
Builds a Codec[M] using the supplied function M => A. The returned codec uses the original
Codec[L] for encoding and decoding. When encoding, it computes an A value using the supplied
function and inserts the computed A in to the HList M, yielding an HList L. That HList L
is then encoded using the original codec.
Companion for DeriveHListElementAux.