Make it so that

(A) All modules imported by Prelude are PrelXXX modules,
    not library modules (notably Ix, Monad were culprits).

    This lines up with the Hugs story, and is more intuitive.

(B) All things needed implicitly by syntax (e.g. do-notation
    needs Monad) come from PrelXXX modules, even if they aren't
    visible when you say 'import Prelude'.

These changes simplify the story, and fix the 'looking for [boot]
interface for Ix' problem.


This change is not quite complete.  I'm committing it so
Simon can finish it off.