MUL instruction execution tested successfully.
MUL instruction execution added.
Removed SK (skip) instruction (keeping SKZ; skip if zero). Unconditional skip is unlikely to be very useful. HALT opcode added; an instruction to signal the end of a program.
Adjusted constructor constraints on opcode values after removal of SK opcode.
Added Halt instruction.
Updated instruction test in light of removal of SK instruction, and the restrictions placed on fields of arithmetic instructions.
Added BR execution.
Tests execution of BR instruction successfully.