Note [Un-unified unification variables]

What should we do if we find a Flexi unification variable?
There are three possibilities:

* DefaultFlexi: this is the common case, in situations like
     length @alpha ([] @alpha)
  It really doesn't matter what type we choose for alpha.  But
  we must choose a type!  We can't leave mutable unification
  variables floating around: after typecheck is complete, every
  type variable occurrence must have a binding site.

  So we default it to 'Any' of the right kind.

  All this works for both type and kind variables (indeed
  the two are the same thing).

* SkolemiseFlexi: is a special case for the LHS of RULES.
  See Note [Zonking the LHS of a RULE]

* RuntimeUnkFlexi: is a special case for the GHCi debugger.
  It's a way to have a variable that is not a mutable
  unification variable, but doesn't have a binding site
  either.

* NoFlexi: See Note [Error on unconstrained meta-variables]
  in GHC.Tc.Utils.TcMType. This mode will panic on unfilled
  meta-variables.