Return results from parse_b / parse_signless_b directly in parse_nth … #414

Job	Run time
Linux (nightly, --features malloc_size_of)	2m 15s
Linux (nightly)	2m 9s
Linux (beta)	33s
Linux (beta, --features malloc_size_of)	33s
Linux (beta, --features dummy_match_byte)	33s
Linux (nightly, --features bench,dummy_match_byte)	5m 16s
Linux (nightly, --features dummy_match_byte)	2m 10s
Linux (1.68.0, --features malloc_size_of)	5m 31s
Linux (1.68.0, --features dummy_match_byte)	5m 43s
Linux (stable)	32s
Linux (stable, --features dummy_match_byte)	34s
Linux (nightly, --features bench)	5m 16s
Linux (1.68.0)	5m 33s
Linux (stable, --features malloc_size_of)	38s
Result	2s
	37m 18s

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