Remove embed method, it is useless to have both that and reassemble
Delta.scala: minimize dependencies on Scalaz to ease porting

Next commit will switch to Scalaz 7, and then I'll rebuild the Delta framework.
Prepare for that by removing dependencies on more esoteric Scalaz components, which changed among releases.
Switch to Scalaz 7

This required mostly changes elsewhere, in TransformationCombinators.
Define interface for minimal encoding of Delta groupoids

Next step: try implementing it.