Implement `product_axis` #105

Job	Run time
cargo-careful	0s
docs/${{ matrix.rust }}	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
clippy/beta	22s
format/nightly	13s
tests/stable	36s
tests/beta	37s
tests/nightly	20s
tests/1.57.0	37s
conclusion	1s
	2m 46s

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