Add left-to-right impredicative instantiation

Add left-to-right impredicative instantiation

People keep complaining, with some justification, that

	runST $ foo

doesn't work.  So I've finally caved in.  The difficulty with the above
is that we need to decide how to instantiate ($)'s type arguments based
on the first argument (runST), and then use that info to check the second
argumnent.  There is a left-to-right flow of information.

It's not hard to implement this, and it's clearly useful.  The main
change is in TcExpr.tcArgs, with some knock-on effects elsewhere.

I was finally provoked into this by Trac #981, which turned out, after some
head-scratching, to be another instance of the same problem.

(There was some bug-fixing too; a type like ((?x::Int) => ...) is a polytype
even though it has no leading for-alls, but the new TcUnify code was not
treating it right.)

Test for this is tc222