api: Add as_match method #2426

Job	Run time
testfull-regex-automata	9m 14s
test (stable, ubuntu-latest, stable)	4m 14s
testfull-regex	4m 7s
msrv	1m 2s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 54s
testfull-regex-capi	24s
testfull-regex-syntax	2m 18s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 39s
miri-regex-automata	14m 15s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 1s
test (beta, ubuntu-latest, beta)	3m 44s
rustfmt	14s
test (nightly, ubuntu-latest, nightly)	2m 56s
test (macos, macos-latest, stable)	3m 4s
test (win-msvc, windows-latest, stable)	7m 8s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 28s
	1h 15m 42s

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