feat: support no_std #17

Job	Run time
clippy	55s
feature-checks	11s
docs	34s
fmt	11s
miri --no-default-features	47s
test stable --no-default-features	8s
miri	49s
test stable	12s
miri --all-features	53s
test stable --all-features	33s
test beta --no-default-features	22s
test beta	25s
test beta --all-features	33s
test nightly --no-default-features	27s
test nightly	24s
test nightly --all-features	32s
test 1.65 --no-default-features	23s
test 1.65	16s
test 1.65 --all-features	35s
	9m 10s

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