chore: replace `bitvec` with `fixedbitset` #281

Job	Run time
Build and test (miri, nightly)	1m 20s
msrv	31s
Build and test (ubuntu-latest, stable)	1m 10s
Checking clippy, fmt and docs	33s
Build and test (windows-latest, stable)	3m 21s
semver-checks	21s
Build and test (macOS-latest, stable)	1m 29s
	8m 45s

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