Changes to ForkTable.get_mut()
Changes to ForkTable.get_mut()

 ForkTables now only allow mutable references to entries in their
level. This is so that we don’t have to keep a mutable reference to the
parent (as this was leading to some headaches).

TODO: attempting to modify a key that’s defined in a  lower level
should overwrite it at this level; figure this out.

hawkw/seax,hawkw/seax