feat(mask): unset bit, count set bits, index of the first set bit (#58) #179

Job	Run time
clippy	37s
miri --no-default-features	4m 42s
feature-checks	45s
miri	4m 42s
test stable --no-default-features	30s
miri --all-features	4m 56s
test stable	29s
docs	28s
test stable --all-features	44s
test beta --no-default-features	34s
fmt	13s
test beta	36s
test beta --all-features	47s
test nightly --no-default-features	36s
test nightly	39s
test nightly --all-features	50s
test 1.79 --no-default-features	18s
test 1.79	19s
test 1.79 --all-features	28s
	23m 13s

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