Expand opcodes and reduce subops

[EdwardLarson]

Since opcodes are now a full byte, many more can be supported. And since there is now direct dispatch between instructions, but switch statements between subops, moving subop functionality to opcodes will likely result in a performance improvement. Example: Splitting arithmetic instructions into Number and Rational versions, rather than handling it in a subop.
Eventually the goal is to all but eliminate branches (loops, switches, conditionals) within instruction code.