------------------------------------------------
	Dramatically improve the error messages arising
	from failed unifications triggered by 'improvement'
	------------------------------------------------

A bit more plumbing in FunDeps, and consequential wibbles elsewhere

Changes this:

    Couldn't match `Int' against `[(String, Int)]'
	Expected type: Int
	Inferred type: [(String, Int)]

to this:

    Foo.hs:8:
	Couldn't match `Int' against `[(String, Int)]'
	    Expected type: Int
	    Inferred type: [(String, Int)]
	When using functional dependencies to combine
	  ?env :: Int, arising from a type signature at Foo.hs:7
	  ?env :: [(String, Int)],
	    arising from use of implicit parameter `?env' at Foo.hs:8
	When generalising the types for ident