Refactor ADD, SUB, and END classes
Refactor instructions with 3 operands
Refactor Instruction and SPC classes
Refactor SHF and BRN instructions
Refactor Instructions to use word_length method
Refactor 2 operand instructions

HBY LBY LOD STR NOT inherit common code from Instruction2 class
Refactor 2 operand instructions even further
Move END_ class with other classes
Refactor Instruction2 and Instruction3 classes
Update to-do list
Update to-do list