another revamp of unit conversion (rpn 5.19.alpha1)

After thinking about it for 2 weeks, I realized the real issue with unit
conversion is that length, area, and volume are weirdly interchangeable
when you start multiplying them together.  I don't know if any other
unit types have this problem.

Therefore, if I treat length, area and volume specially, the conversions
go a lot more easily.

I'm sure I'll find other problems, so this is considered alpha for now.
rpn 5.16.alpha2

Minor bug fixes