Implement CHIP-8 Instructions: Arithmetic

[fokoid]

• [x] 0x7XNN Adds NN to VX (carry flag is not changed). (Vx += NN)
• [x] 0x8XY4 Adds VY to VX. VF is set to 1 when there's a carry, and to 0 when there is not. (Vx += Vy)
• [x] 0x8XY5 VY is subtracted from VX. VF is set to 0 when there's a borrow, and 1 when there is not. (Vx -= Vy)
• [x] 0x8XY7 Sets VX to VY minus VX. VF is set to 0 when there's a borrow, and 1 when there is not. (Vx = Vy - Vx)

Pseudocode and descriptions were sourced from the CHIP-8 Wikipedia article and are licensed under the Creative Commons Attribution-ShareAlike License 3.0.