-------------------------------------
      Add Core Notes and the {-# CORE #-} pragma
	-------------------------------------

This is an idea of Hal Daume's. The key point is that Notes in Core
are augmented thus:

  data Note
    = SCC CostCentre
    | ...
    | CoreNote String     -- NEW

These notes can be injected via a Haskell-source pragma:

   f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)

This wraps a (Note (CoreNote "foo")) around the 'show' variable,
and a similar note around the argument to 'show'.

These notes are basically ignored by GHC, but are emitted into
External Core, where they may convey useful information.

Exactly how code involving these notes is munged by the simplifier
isn't very well defined.  We'll see how it pans out.  Meanwhile
the impact on the rest of the compiler is minimal.