serializer: Simplify a bit number serialization. #409

Job	Run time
Linux (nightly, --features dummy_match_byte)	2m 9s
Linux (nightly)	2m 15s
Linux (1.68.0, --features malloc_size_of)	2m 33s
Linux (stable, --features malloc_size_of)	41s
Linux (beta)	35s
Linux (stable, --features dummy_match_byte)	37s
Linux (beta, --features malloc_size_of)	34s
Linux (1.68.0)	2m 29s
Linux (beta, --features dummy_match_byte)	38s
Linux (nightly, --features malloc_size_of)	2m 10s
Linux (1.68.0, --features dummy_match_byte)	2m 42s
Linux (nightly, --features bench)	5m 18s
Linux (stable)	38s
Linux (nightly, --features bench,dummy_match_byte)	5m 15s
Result	4s
	28m 38s

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