Simon's major commit to re-engineer the constraint solver

The driving change is this:

* The canonical CFunEqCan constraints now have the form
       [G] F xis ~ fsk
       [W] F xis ~ fmv
  where fsk is a flatten-skolem, and fmv is a flatten-meta-variable
  Think of them as the name of the type-function application

See Note [The flattening story] in TcFlatten.  A flatten-meta-variable
is distinguishable by its MetaInfo of FlatMetaTv

This in turn led to an enormous cascade of other changes, which simplify
and modularise the constraint solver.  In particular:

* Basic data types
    * I got rid of inert_solved_funeqs altogether. It serves no useful
      role that inert_flat_cache does not solve.

    * I added wl_implics to the WorkList, as a convenient place to
      accumulate newly-emitted implications; see Note [Residual
      implications] in TcSMonad.

    * I eliminated tcs_ty_binds altogether. These were the bindings
      for unification variables that we have now solved by
      unification.  We kept them in a finite map and did the
      side-effecting unification later.  But in cannonicalisation we
      had to look up in the side-effected mutable tyvars anyway, so
      nothing was being gained.

      Our original idea was that the solver would be pure, and would
      be a no-op if you discarded its results, but this was already
      not-true for implications since we update their evidence
      bindings in an imperative way.  So rather than the uneasy
      compromise, it's now clearly imperative!

* I split out the flatten/unflatten code into a new module, TcFlatten

* I simplified and articulated explicitly the (rather hazy) invariants
  for the inert substitution inert_eqs.  See Note [eqCanRewrite] and
  See Note [Applying the inert substitution] in TcFlatten

* Unflattening is now done (by TcFlatten.unflatten) after solveFlats,
  before solving nested implications.  This turned out to simplify a
  lot of code. Previously, unflattening was done as part of zonking, at
  the very very end.

    * Eager unflattening allowed me to remove the unpleasant ic_fsks
      field of an Implication (hurrah)

    * Eager unflattening made the TcSimplify.floatEqualities function
      much simpler (just float equalities looking like a ~ ty, where a
      is an untouchable meta-tyvar).

    * Likewise the idea of "pushing wanteds in as givens" could be
      completely eliminated.

* I radically simplified the code that determines when there are
  'given' equalities, and hence whether we can float 'wanted' equalies
  out.  See TcSMonad.getNoGivenEqs, and Note [When does an implication
  have given equalities?].

  This allowed me to get rid of the unpleasant inert_no_eqs flag in InertCans.

* As part of this given-equality stuff, I fixed Trac #9211. See Note
  [Let-bound skolems] in TcSMonad

* Orientation of tyvar/tyvar equalities (a ~ b) was partly done during
  canonicalisation, but then repeated in the spontaneous-solve stage
  (trySpontaneousSolveTwoWay). Now it is done exclusively during
  canonicalisation, which keeps all the code in one place.  See
  Note [Canonical orientation for tyvar/tyvar equality constraints]
  in TcCanonical

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