Doc
Add t.f. Wrap
Remove unused extensions
First attempt to write more general function

I't doesn't quite work since this approch do not allow to write
functions like:

> wrap :: (forall a. a -> f a) -> ContVec xs -> ContVec (Wrap f xs)

Tag data type needs to be change on each iteration step. Since it contain
Fun being transformed but type class require it to stay the same.
Probably I'll have to stick with one type-class per function approach.
oops
Doc
Add notice
Add another type class extending Arity
Reorder
Remove sequence_ it's too ugly

In particular type signature is troubling. It leaks tag data type outside
and fails to describe that all elemets have same outer type constructor
Add HVector instance to ContVec
Add sequence{A,}
Add wrap/unwrap functions
Clean up export lists and reorder code