chore: replace `bitvec` with `fixedbitset` #279

Job	Run time
Checking clippy, fmt and docs	23s
Build and test (miri, nightly)	1m 18s
msrv	33s
Build and test (ubuntu-latest, stable)	1m 20s
semver-checks	2m 40s
Build and test (windows-latest, stable)	2m 46s
Build and test (macOS-latest, stable)	58s
	9m 58s

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