The type of fork# is over general

When working eta-expansion and reduction, found that fork# had a weaker strictness signature than it should have. In particular, it didn't record that it applies its argument exactly once. Moreover, it has a very strange type:

primop  ForkOp "fork#" GenPrimOp
   a -> State# RealWorld -> (# State# RealWorld, ThreadId# #)
   with
   has_side_effects = True
   out_of_line      = True

It should look like this:

primop  ForkOp "fork#" GenPrimOp
   (State# RealWorld -> (# State# RealWorld, a #))
   -> State# RealWorld -> (# State# RealWorld, ThreadId# #)
   with
   has_side_effects = True
   out_of_line      = True
   strictness  = { \ _arity -> mkClosedDmdSig [ lazyApply1Dmd
                                              , topDmd ] topDiv }

Patch coming.