syntax: optimize some functions in IntervalSet. #1767

Job	Run time
msrv	1m 48s
testfull-regex-automata	19m 26s
rustfmt	7s
miri-regex-automata	11m 44s
test (stable, ubuntu-latest, stable)	6m 41s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	4m 41s
test (stable-mips, ubuntu-latest, stable, mips64-unknown-linux-gnuabi64)	10m 36s
test (beta, ubuntu-latest, beta)	6m 58s
test (nightly, ubuntu-latest, nightly)	8m 22s
test (macos, macos-latest, stable)	10m 47s
test (win-msvc, windows-latest, stable)	10m 34s
test (win-gnu, windows-latest, stable-x86_64-gnu)	10m 3s
testfull-regex	8m 52s
testfull-regex-syntax	3m 17s
testfull-regex-capi	39s
	1h 54m 35s

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