simplifyTop, code beautification etc. Important things:

(a) New top-level defaulting plan, gotten rid of the SimplContext field.
    See Note [Top-level Defaulting Plan]

(b) Serious bug fix in the floatEqualities mechanism
    See Note [Extra TcS Untouchables],[Float Equalities out of Implications]

The changes are mostly confined in TcSimplify but there is a
simplification wave affecting other modules as well.