Bump fixedbitset from 0.4.2 to 0.5.6 #4916

Job	Run time
Check	28s
coverage	7m 42s
Lints (beta)	41s
test (beta)	1m 16s
test_all_features	1m 22s
Lints (stable)	35s
test (stable)	1m 4s
Check if cbindgen runs cleanly for generating the C headers	51s
test (windows)	3m 29s
minimum_rust_version	7m 3s
Publish (dry-run)	8m 14s
test (macos)	1m 31s
Run tests under wasm32-wasi	1m 9s
Check if wasm-pack builds a valid package for the sourmash crate	1m 32s
	36m 57s

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