/// (ExA1) Skip next instruction if key with value Vx not pressed.
(ExA1) Skip next instruction if key with value Vx not pressed.
( ExA1 ) Skip next instruction if key with value Vx not pressed .
fn opcode_sknp(&mut self) {
        let vx = self.v_reg[self.opcode.x()];
        if self.keypad[vx as usize] == 0 {
            self.pc += 2;
        }
        self.pc += 2;
    }
fn opcode_sknp ( & mut self ) { let vx = self . v_reg [ self . opcode . x ( ) ] ; if self . keypad [ vx as usize ] == 0 { self . pc += 2 ; } self . pc += 2 ; }
(ExA1) Skip next instruction if key with value Vx not pressed.
( ExA1 ) Skip next instruction if key with value Vx not pressed .
&mut self