Support compact instructions

[adumont]

For memory instructions, use the two unused bits to encode 3 values of the operand, so the instructions can be written in only 1 byte (instead of two that it should normally be)

For example COPYTO 0 (or 1 or 2) would fit in only 1 byte, encoding the operand (0, 1, or 2) in the two unused bits of the opcode.

That involves changes in the FSM (faster as we bypass the steps of reading the Operand).

Also involves changes in the assembler obviously.