Make it so that Local (i.e. non-top-level) names record
whether they originally came from an interface file.  This
means that when unifying two type variables we can readily
choose one that occurred in the source, rather than one
imported from an interface file.  That in turn improves 
compiler error messages.  E.g.

  rd :: (RealFloat a, RealFrac b) => b -> Transformation a
  rd degrees  = r ((degrees / 180.0) * pi)

used to say

    Could not deduce `Floating a'
	(arising from use of `pi' at Foo.hs:11)
	from the context: (RealFloat a1, RealFrac a)
    Probable cause: missing `Floating a' in type signature for `rd'

[here the 'a' came from the signature for 'pi' in PrelBase;
 the 'a1' is a renamed version of the 'a' in the source pgm]

but now says

    Could not deduce `Floating b'
	(arising from use of `pi' at Foo.hs:11)
	from the context: (RealFloat a, RealFrac b)
    Probable cause: missing `Floating b' in type signature for `rd'