automata: add internal HalfMatch APIs for NFA engines #1799

Job	Run time
msrv	2m 26s
test (stable, ubuntu-latest, stable)	6m 59s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	4m 52s
test (stable-mips, ubuntu-latest, stable, mips64-unknown-linux-gnuabi64)	14m 0s
test (beta, ubuntu-latest, beta)	6m 52s
test (nightly, ubuntu-latest, nightly)	8m 23s
test (macos, macos-latest, stable)	7m 47s
test (win-msvc, windows-latest, stable)	10m 18s
test (win-gnu, windows-latest, stable-x86_64-gnu)	7m 22s
miri-regex-automata	12m 14s
testfull-regex	7m 4s
rustfmt	17s
testfull-regex-automata	17m 54s
testfull-regex-capi	44s
testfull-regex-syntax	4m 38s
	1h 51m 50s

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