Warning
Move Fun & HVector to separate module
Add CPS-encoded vectors
More functions for ContVec
Warnings
Remove monadic version of ContVec. It siplifies things
Use ContVec as intermediate
Implement concat
Implement mk# in terms of ContVec
prototype of isomorphism
Add type class Arity

It's very similar to one used in fixed-vector. It simplifies API a lot.

 * ConstF is gone
 * Constraints "Functor (Fun xs)" are replaced with "Arity xs"