ActualXID canonicalization

[Eric Dean Tribble <tribble>]

When EXIDs come in from the disk, the equivalent pairID is looked up
in a GrandMap table to find if a corresponding EXID already exists.
If it does, return it, otherwise create a new one.  This behavior
strikes me as necessary anytime we create an EXID.  Therefore it
should be moved into a pseudo constructor, and the pseudo constructor
should be called by the ActualXIDRecipe.  Then all the
canonicalization is encapsulated nice and cleanly.  It may even be
that the canonicalization table can be per backend, and so in a static
table in the module.  Any takers?

I'll do this as soon as I get teh intermediate image.

dean