Added two useful opcode mnemonic aliases.

`bnz` -- jump if not zero.
`bze` -- jump if zero.

Those mnemonics are useful in some code contexts, such as loops:
```asm
    ldx #0
loop:
    lda string,x
    bze exit
    sta buffer,x
    inx
    bnz loop
exit:
    rts
```
Their meanings make more sense there than the original mnemonics, `beq` and `bne`, do.