Better handling of dividing 0 by a Literal

Fixed the special case handling so that dividing a 0 without units by a number with units results in a Literal without units
Fixed equality with 0-without-units

Checking if a number is equal to zero is a common way to avoid dividing by zero. Previously, 0.units would compare as unequal to 0, which would cause errant divide-by-zero errors. Added a special case to catch the situation and do the right thing.
Improved testing of subtraction involving 0