Make unsafePerformIO lazy

Make unsafePerformIO lazy

The stricteness analyser used to have a HACK which ensured that NOINLNE things
were not strictness-analysed.  The reason was unsafePerformIO. Left to itself,
the strictness analyser would discover this strictness for unsafePerformIO:
	unsafePerformIO:  C(U(AV))
But then consider this sub-expression
	unsafePerformIO (\s -> let r = f x in
			       case writeIORef v r s of (# s1, _ #) ->
			       (# s1, r #)
The strictness analyser will now find that r is sure to be eval'd,
and may then hoist it out.  This makes tests/lib/should_run/memo002
deadlock.

Solving this by making all NOINLINE things have no strictness info is overkill.
In particular, it's overkill for runST, which is perfectly respectable.
Consider
	f x = runST (return x)
This should be strict in x.

So the new plan is to define unsafePerformIO using the 'lazy' combinator:

	unsafePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)

Remember, 'lazy' is a wired-in identity-function Id, of type a->a, which is
magically NON-STRICT, and is inlined after strictness analysis.  So
unsafePerformIO will look non-strict, and that's what we want.