---------
		Main.main
		---------

A bunch of related fixes concerning 'main'

* Arrange that 'main' doesn't need to be defined in module Main;
  it can be imported.

* The typechecker now injects a binding
	Main.$main = PrelTopHandler.runMain main

  So the runtime system now calls Main.$main, not PrelMain.main.
  With z-encoding, this look like
		Main_zdmain_closure

* The function
  	PrelTopHandler.runMain :: IO a -> IO ()
  wraps the programmer's 'main' in an exception-cacthing wrapper.

* PrelMain.hs and Main.hi-boot are both removed from lib/std, along
  with multiple lines of special case handling in lib/std/Makefile.
  This is a worthwhile cleanup.

* Since we now pick up whatever 'main' is in scope, the ranamer gets
  in on the act (RnRnv.checkMain).  There is a little more info to
  get from the renamer to the typechecker, so I've defined a new type
  Rename.RnResult (c.f. TcModule.TcResult)

* With GHCi, it's now a warning, not an error, to omit the binding
  of main (RnEnv.checkMain)

* It would be easy to add a flag "-main-is foo"; the place to use
  that information is in RnEnv.checkMain.


-------

On the way I made a new type,
	type HscTypes.FixityEnv = NameEnv Fixity
and used it in various places I'd tripped over