add Codec.emap

allows validation of data passing through codec
adjust per the review feedback
Remove old mima exclusions
Simplified coproduct codec implementation and added choice codec support for coproduct and non-coproduct codecs
Fix string interpolation in coproduct codec
Minor internal syntax cleanup in coproduct codecs
Merge pull request #20 from searler/either_branch

Add combinator that allows validation of data being encoded or decoded
Add emap and econtramap to Decoder and Encoder
Add overrides for encoder/decoder transforms to GenCodec
Fixes #22. Adds encodeOnly and decodeOnly combinators
Add overloads for encodeOnly/decodeOnly that work nicer with inference
Improve inference of encodeOnly