added equzero (same as cmpzero but only outputs Zero flag)

[ZERICO2005]

__*equzero is identical to __*cmpzero except that it only outputs to the Zero flag. Currently it just aliases __*cmpzero, but the intent is that it could maybe link to a smaller/faster routine.

__i48cmpzero:
; CC: 13 bytes
; Minimum  :  8F + 3R + 2
; arg == 0 : 11F + 3R + 3
; Maximum  : 14F + 3R + 3
	ex	de, hl
	add	hl, de
	or	a, a
	sbc	hl, de
	ex	de, hl
	ret	nz
	sbc	hl, de
	ret	p
	inc	e
	dec	de
	ret

; implementation 1
__i48equzero:
; CC: 9 bytes
; Minimum  :  6F + 3R + 2
; arg == 0 : 10F + 3R + 2
; Maximum  : 10F + 3R + 2
	add	hl, de
	or	a, a
	sbc	hl, de
	ret	nz
	sbc	hl, de
	add	hl, de
	ret

; implementation 2
__i48equzero:
; CC: 10 bytes
; Minimum  :  6F + 3R + 2
; arg == 0 : 11F + 3R + 2
; Maximum  : 11F + 3R + 2
	or	a, a
	sbc	hl, de
	add	hl, de
	ret	nz
	sbc	hl, hl
	adc	hl, de
	ret

[ZERICO2005]

yeah previously this commit also did have __*cmpequ as well with the same sort of idea of being __*cmpu but only outputting to the Z flag. But that is a much more niche optimization and likely not worth implementing into the compiler.