Fix one of the bugs in previous commit's test-case

This was done by simply removing the entry-points (uoverload_on, and
uset_fixity) that were causing problems. They have long been redundant
in terms of their desired effect, and introduced buggy behaviour with
the switch to explicit grammar deltas.

The uoverload_on function is still used internally.

Add test demonstrating bugs in some unicode fns

In particular, the grammar-deltas embodied in these effect are not
recorded against the theory and so merges involving that grammar won't
pick up the Unicode forms of the rules.