Another go at the handling of -< in arrow notation, continuing and
fixing the changes in

	http://www.haskell.org/pipermail/cvs-all/2005-April/040391.html

Now do the same thing in the renamer as we do in the type checker,
i.e. return to the environment of the proc when considering the left
argument of -<.

This is much simpler than the old proc_level stuff, and matches the
type rules more clearly.  But there is a change in error messages.
For the input

	f :: Int -> Int
	f = proc x -> (+x) -< 1

GHC 6.4 says

	test.hs:6:
	    Command-bound variable `x' is not in scope here
		Reason: it is used in the left argument of (-<)
	    In the second argument of `(+)', namely `x'
	    In the command: (+ x) -< 1
	    In the definition of `f': f = proc x -> (+ x) -< 1

but now we just get the blunt

	test.hs:6:16: Not in scope: `x'

The beauty is all on the inside.

Similarly leakage of existential type variables (arrow1) is detected,
but the error message isn't very helpful.