Mark _ as unreachable

Rust's exhaustiveness checking is based entirely on enums and never
values, so this fails because u8 simply isn't an enum type.

    error[E0004]: non-exhaustive patterns: `_` not covered
      --> src/main.rs:39:26
       |
    39 |     let op_bytes = match op_code {
       |                          ^^^^^^^ pattern `_` not covered

rust-lang/rust#12483

src/main.rs

@@ -293,7 +293,7 @@ fn disassemble(pc: u16, buffer: &Vec<u8>) -> u16 {
         0xfd => { not_implemented() },
         0xfe => { disassemble_2("CPI", buffer[pc + 1]) },
         0xff => { disassemble_1("RST 7") },
-           _ => { not_implemented() }
+           _ => unreachable!()
     };
 
     op_bytes