GADTs, GADTs everywhere!
Decompose field parameter types and mkVinFormat using
VinFieldParameter class.

Reverted GADTs in VFAccessor datatype (somewhy vinnie fails to
typecheck when GADT syntax is used)
Move mkAcc out of VinFieldParameter class
Rename NameTitles → Titles
ParamAcc associated data family (existential wrapper for VinFormat accessors)