Fix is_ident_start signature to take immutable reference #367

Job	Run time
Linux (nightly, --features dummy_match_byte)	2m 14s
Linux (beta, --features dummy_match_byte)	54s
Linux (beta, --features malloc_size_of)	35s
Linux (nightly, --features malloc_size_of)	2m 17s
Linux (nightly)	2m 13s
Linux (stable)	35s
Linux (1.66.0, --features malloc_size_of)	3m 2s
Linux (nightly, --features bench,dummy_match_byte)	5m 20s
Linux (beta)	32s
Linux (1.66.0)	3m 7s
Linux (stable, --features malloc_size_of)	36s
Linux (1.66.0, --features dummy_match_byte)	3m 16s
Linux (stable, --features dummy_match_byte)	31s
Linux (nightly, --features bench)	5m 16s
Result	3s
	30m 31s

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