Provide more options for futzing with ='s precedence

Now there are four functions:

  {temp,}_{tight,loose}_equality

The temp versions create an effect that is not exported in the theory
file, and can be called before new_theory without causing annoying
error messages about changes being discarded.

The tight precedence is 450, which is the same level as the other
standard relational operators (e.g., <). The loose precedence is 100,
which is historical, and clearly appropriate for = construed as iff.
Fiddle with datatype printing in TeX (with selftest of sorts)