rpn 5.15.0

Converted the integral constant operators (million, etc.) to operators
so you don't have to explicitly multiply by them.  The 'constant' unit
types are immediate converted to plain numbers, so they don't really
work like units.  They don't work with 'convert' yet, but I'll fix that.

Added 'dhms' operator for days, hours, minutes, seconds.

Some day I need to tackle adding help for the units...