chore: implement `is_zero` in terms of `AcirVar`s #1209

Job	Run time
build-nargo	2m 16s
build-acvm-js	3m 21s
build-noirc-abi	3m 24s
build-noir-wasm	4m 57s
ACVM JS (Node.js)	34s
ACVM JS (Browser)	1m 25s
noir_codegen	1m 2s
noirc_abi	49s
noir-js-backend-barretenberg	46s
Noir JS	1m 31s
noir_wasm	48s
Integration Tests	8m 33s
End	1s
	29m 27s

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