Improve the GHCi interaction

		Merge to STABLE?

This fix addresses Sourceforge #1156554 "GHCi: No instance for (Show (IO ()))",
and simultaneously improves the top-level interaction in two other ways:

- Only one error can show up (previously there could be two)

- If an I/O action gives a Showable result, the result is printed
  (provided it isn't ()).  So
	prompt> return 4
  prints 4, rather than nothing

- For command-line 'let' and 'x<-e' forms, if exactly one variable
  is bound, we print its value if it is Showable and not ()
	prompt> let x = 4
	4
	prompt> x <- return 5
	5