syntax: optimize some functions in IntervalSet. #1774

Job	Run time
msrv	1m 45s
miri-regex-automata	9m 50s
rustfmt	12s
test (stable, ubuntu-latest, stable)	6m 50s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	5m 35s
test (stable-mips, ubuntu-latest, stable, mips64-unknown-linux-gnuabi64)	11m 3s
test (beta, ubuntu-latest, beta)	8m 14s
test (nightly, ubuntu-latest, nightly)	8m 16s
test (macos, macos-latest, stable)	9m 27s
test (win-msvc, windows-latest, stable)	13m 15s
test (win-gnu, windows-latest, stable-x86_64-gnu)	6m 3s
testfull-regex	6m 58s
testfull-regex-automata	14m 42s
testfull-regex-capi	40s
testfull-regex-syntax	3m 47s
	1h 46m 37s

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