Move LayoutBitset to its own module #582

Job	Run time
Pass MSRV values to other jobs	2s
format/nightly	14s
cargo-careful	1m 41s
nostd/thumbv6m-none-eabi/stable	23s
miri	9m 58s
cross_test/i686-unknown-linux-gnu/stable	1m 54s
cross_test/s390x-unknown-linux-gnu/stable	8m 19s
docs/nightly	22s
clippy/stable	22s
blas-msrv	45s
tests/beta	2m 23s
tests/1.64.0	3m 18s
tests/stable	2m 35s
tests/nightly	2m 45s
conclusion	4s
	35m 5s

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