Add callback-oriented versions of `constraint_system` and `generate_witness` #438

	name: ci

	on:
	[pull_request]

	jobs:
	docker-build:
	runs-on: ubuntu-latest
	steps:
	-
	name: Checkout
	uses: actions/checkout@v2
	-
	uses: docker/setup-buildx-action@v1
	id: docker-build
	with:
	install: true
	-
	name: Build
	run: \|
	docker build . # will run buildx

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