Note [quantifyTyVars]

quantifyTyVars is given the free vars of a type that we
are about to wrap in a forall.

It takes these free type/kind variables (partitioned into dependent and
non-dependent variables) skolemises metavariables with a TcLevel greater
than the ambient level (see Note [Use level numbers for quantification]).

* This function distinguishes between dependent and non-dependent
  variables only to keep correct defaulting behavior with -XNoPolyKinds.
  With -XPolyKinds, it treats both classes of variables identically.

* quantifyTyVars never quantifies over
    - a coercion variable (or any tv mentioned in the kind of a covar)
    - a runtime-rep variable