Note [Checking partial type signatures]

This Note is about tcHsPartialSigType.  See also
Note [Recipe for checking a signature]

When we have a partial signature like
   f :: forall a. a -> _
we do the following

* tcHsPartialSigType does not make quantified type (forall a. blah)
  and then instantiate it -- it makes no sense to instantiate a type
  with wildcards in it.  Rather, tcHsPartialSigType just returns the
  'a' and the 'blah' separately.

  Nor, for the same reason, do we push a level in tcHsPartialSigType.

* We instantiate 'a' to a unification variable, a TyVarTv, and /not/
  a skolem; hence the "_Tv" in bindExplicitTKBndrs_Tv.  Consider
    f :: forall a. a -> _
    g :: forall b. _ -> b
    f = g
    g = f
  They are typechecked as a recursive group, with monomorphic types,
  so 'a' and 'b' will get unified together.  Very like kind inference
  for mutually recursive data types (sans CUSKs or SAKS); see
  Note [Cloning for type variable binders]

* In GHC.Tc.Gen.Sig.tcUserSigType we return a PartialSig, which (unlike
  the companion CompleteSig) contains the original, as-yet-unchecked
  source-code LHsSigWcType

* Then, for f and g /separately/, we call tcInstSig, which in turn
  call tcHsPartialSig (defined near this Note).  It kind-checks the
  LHsSigWcType, creating fresh unification variables for each "_"
  wildcard.  It's important that the wildcards for f and g are distinct
  because they might get instantiated completely differently.  E.g.
     f,g :: forall a. a -> _
     f x = a
     g x = True
  It's really as if we'd written two distinct signatures.

* Nested foralls. See Note [Levels for wildcards]

* Just as for ordinary signatures, we must solve local equalities and
  zonk the type after kind-checking it, to ensure that all the nested
  forall binders can "see" their occurrences

  Just as for ordinary signatures, this zonk also gets any Refl casts
  out of the way of instantiation.  Example: #18008 had
       foo :: (forall a. (Show a => blah) |> Refl) -> _
  and that Refl cast messed things up.  See #18062.