This renames the arithmetic internal and API kind PLUS to ADD for
consistency with our naming scheme for other operators (e.g.,
BITVECTOR_ADD, FLOATINGPOINT_ADD).
This renames the arithmetic kinds MINUS and UMINUS on the
API level for consistency with our naming scheme for other
operators (e.g., BITVECTOR_SUB, FLOATINGPOINT_SUB).
This PR removes the conversion of rationals to double in favour of properly handling them as rationals (as pairs of integers) in the C++ quickstart example.
