Reduce seqId magic

In MkId.hs we see

Note [seqId magic]
~~~~~~~~~~~~~~~~~~
'GHC.Prim.seq' is special in several ways.

a) In source Haskell its second arg can have an unboxed type
      x `seq` (v +# w)
   But see Note [Typing rule for seq] in TcExpr, which
   explains why we give seq itself an ordinary type
         seq :: forall a b. a -> b -> b
   and treat it as a language construct from a typing point of view.

But nowadays we can handle (a) via levity polymorphism. So we should make its (still wired in) type be

seq :: forall {r::RuntimeRep} (a :: TYPE LiftedRep) (b :: TYPE r). a -> b -> b

Less magic!

---

Second point. In MkId we see

    info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
                       `setUnfoldingInfo` mkCompulsoryUnfolding rhs

Why do we need that alwaysInlinePragma? After all, the Mighty Simplifier always inlines compulsory things:

activeUnfolding mode id
  | isCompulsoryUnfolding (realIdUnfolding id)
  = True   -- Even sm_inline can't override compulsory unfoldings

But, strangely, the Very Simple Optimiser does not:

simple_app env (Var v) as
...
  | let unf = idUnfolding v
  , isCompulsoryUnfolding (idUnfolding v)
  , isAlwaysActive (idInlineActivation v)
    -- See Note [Unfold compulsory unfoldings in LHSs]
  = simple_app (soeZapSubst env) (unfoldingTemplate unf) as

So it is a bit less gung-ho about inlining compulsory unfoldings. Why?

The sole reasons is that simpleOptExpr is applied to the entire LHS of a rule (DsBinds)

 where
   lhs1         = drop_dicts orig_lhs
   lhs2         = simpleOptExpr dflags lhs1  -- See Note [Simplify rule LHS]
   (fun2,args2) = collectArgs lhs2

and we don't want to inline seq here even though it has a compulsory unfolding: see Note [User-defined RULES for seq]. Gah!

Decision: let's narrow the special case in simple_app to focus just on seq, with a suitable pointer:

simple_app env (Var v) as
...
  | let unf = idUnfolding v
  , isCompulsoryUnfolding (idUnfolding v)
  , v `hasKey` seqIdKey   -- See Note XXXXX

And then get rid of the inline-pragma stuff on wired-in Ids with compulsory unfoldings.