Version management

[WARNING: may not work!  Don't update till I've tested it.]

This commit is a first stab at getting version management to
work properly.  The main trick is to get consistent naming when
comparing old and new versions of the same module.  

Some functionality has moved arond between
  coreSyn/CoreTidy, which tidies up the result of
			the middle end of the compiler
	Main change: now responsible for figuring out which
	Ids are "external" (i.e visible to importing modules),
	and constructing the final IdInfo for each Id

  main/MkIface, which produces the ModIface and ModDetails
		for the module being compiled
	Main change: CoreTidy does more, so MkIface does less

  stgSyn/CoreToStg, which converts Core to STG
	Main change: responsible for globalising internal
	names when we are doing object code splitting 
			
The game plan is documented at the top of CoreTidy.