Support for TPTP's TFF0 (with arithmetic)

Support for TPTP's TFF0 (with arithmetic)

This commit reverses an "SZS ontology compliance hack" that was
done for CASC-24 this year, and adds a TPTP pretty-printer which
is capable of outputting results in the TPTP way (rather than the
SMT way).

This commit includes minor changes to the Expr package to add
obvious missing functionality, and to fix the way expressions
with builtin operators are made.  These changes are truly a
_fix_, the implementation had not been properly aligned with
the design vision for some corner cases.