Use HsTyPats in associated type family defaults

Associated type family default declarations behave strangely in a
couple of ways:

1. If one tries to bind the type variables with an explicit `forall`,
   the `forall`'d part will simply be ignored. (#16110)
2. One cannot use visible kind application syntax on the left-hand
   sides of associated default equations, unlike every other form
   of type family equation. (#16356)

Both of these issues have a common solution. Instead of using
`LHsQTyVars` to represent the left-hand side arguments of an
associated default equation, we instead use `HsTyPats`, which is what
other forms of type family equations use. In particular, here are
some highlights of this patch:

* `FamEqn` is no longer parameterized by a `pats` type variable, as
  the `feqn_pats` field is now always `HsTyPats`.
* The new design for `FamEqn` in chronicled in
  `Note [Type family instance declarations in HsSyn]`.
* `TyFamDefltEqn` now becomes the same thing as `TyFamInstEqn`. This
  means that many of `TyFamDefltEqn`'s code paths can now reuse the
  code paths for `TyFamInstEqn`, resulting in substantial
  simplifications to various parts of the code dealing with
  associated type family defaults.

Fixes #16110 and #16356.