parser: Implement recent syntax spec change to avoid parsing custom p… #318

Job	Run time
Linux (nightly)	2m 17s
Linux (nightly, --features dummy_match_byte)	2m 16s
Linux (beta)	54s
Linux (beta, --features dummy_match_byte)	40s
Linux (stable)	46s
Linux (stable, --features dummy_match_byte)	44s
Linux (1.63.0)	1m 58s
Linux (1.63.0, --features dummy_match_byte)	2m 7s
Linux (nightly, --features bench)	3m 57s
Linux (nightly, --features bench,dummy_match_byte)	3m 52s
Result	1s
	19m 32s

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