Make arith substitute its own utility (#8765)

Make arith substitute its own utility (#8765)

Arithmetic substitutions behave differently in two ways:
(1) they traverse only symbols belonging to arithmetic
(2) they allow mixing Int/Real
This makes ArithSubs derive from the more general Subs class with these two behavior changes.

This is one of the last remaining non-trivial steps towards for eliminating TypeNode::isSubtypeOf.