Change the representation and move TcBuiltInSynFamily.

The changes in more detail:

  * `TcBuiltInSynFamily` is now known as `BuiltinSynFamily` and
     lives in `CoAxiom`

  * `sfMatchFam` returns (CoAxiomRule, [Type], Type),
     which is enough to make Coersion or TcCoercion,
     depending on what what we need.

  * The rest of the compiler is updated to reflect these changes,
    most notably we can eliminate the cludge (XXX) in FamInstEnv
    and remove the lhs-boot file.