This patch implements @k-binders introduced in GHC Proposal #425
and guarded behind the TypeAbstractions extension:

	type D :: forall k j. k -> j -> Type
	data D @k @j a b = ...
	       ^^ ^^

To represent the new syntax, we modify LHsQTyVars as follows:

	-  hsq_explicit :: [LHsTyVarBndr () pass]
	+  hsq_explicit :: [LHsTyVarBndr (HsBndrVis pass) pass]

HsBndrVis is a new data type that records the distinction between
type variable binders written with and without the @ sign:

	data HsBndrVis pass
	  = HsBndrRequired
	  | HsBndrInvisible (LHsToken "@" pass)

The rest of the patch updates GHC, template-haskell, and haddock
to handle the new syntax.

Parser:
  The PsErrUnexpectedTypeAppInDecl error message is removed.
  The syntax it used to reject is now permitted.

Renamer:
  The @ sign does not affect the scope of a binder, so the changes to
  the renamer are minimal.  See rnLHsTyVarBndrVisFlag.

Type checker:
  There are three code paths that were updated to deal with the newly
  introduced invisible type variable binders:

    1. checking SAKS: see kcCheckDeclHeader_sig, matchUpSigWithDecl
    2. checking CUSK: see kcCheckDeclHeader_cusk
    3. inference: see kcInferDeclHeader, rejectInvisibleBinders

  Helper functions bindExplicitTKBndrs_Q_Skol and bindExplicitTKBndrs_Q_Tv
  are generalized to work with HsBndrVis.

Updates the haddock submodule.

Metric Increase:
    MultiLayerModulesTH_OneShot

Co-authored-by: Simon Peyton Jones <simon.peytonjones@gmail.com>