Revert tcInstTyCoVas --> tcInstTyVars
Re-tune tcInstTyCoVars and friends to deal with coercions.
Revert "Add the closed type family `Cast`."

This reverts commit 09ba9794bd2ff61c85b38ff9ca7afc1bbff68fab.
Teach desugarer about unboxed coercions
Stage 1 compiles.