Main changes were to tp_eval.P in compiler so that constant
functions could be defined to the compiler within arithmetic
expressions. Using this change, I added pi/0 and e/0 to the
compiler (they are already in the evaluator). These changes are
quite local, and should not affect arithmetic changes that David
might make (I hope).
Also, while I was there, I tweaked an error message or two to
make their messages more comprehensible.