Make Unroll an open type family

[Unisay]

This PR contains an alternative approach to un-rolling AsData-types: it uses an open type family for it.

This has its own pro:

• Users are free to specify how to unroll any type (including custom AsData types)
  by provinding a TF instance, for example:
  type instance Unroll AsDataFoo = '[AsDataFoo, Bool, Integer]

and con:

• There is no default un-rolling of generic types, instead users need to instantiate the Unroll TF for every custom data type:
  type instance Unroll MyGenericFoo = GenericUnroll Foo

[Unisay]

Decided not to implement it.