Break up TyCoRep

The TyCoRep module is one of the largest modules in GHC and mixes a variety of concerns. This makes identifying the reasons for the many import cycles that it participates in quite challenge. While attempting an implementation of a new free type/coercion variables traversal I ran head-long into one such cycle.

This patch breaks up TyCoRep along the boundaries of its various concerns:

• TyCoRep: Contains the Coercion, Type, and related type definitions and a few simple predicates but nothing further

• TyCoPpr: Contains the the pretty-printer logic

• TyCoFVs: Contains the free variable computations (and tyConAppNeedsKindSig, although I suspect this should change)

• TyCoSubst: Contains the substitution logic for types and coercions

• TyCoTidy: Contains the tidying logic for types

While we are able to eliminate a good number of SOURCE imports (and make a few others smaller) with this change, we must introduce one new hs-boot file for TyCoPpr so that TyCoRep can define Outputable instances for the types it defines.

In addition, we move the well-scoped free variable logic to TyCoFVs to eliminate the need for some source imports.