capi: add split #2378

Job	Run time
testfull-regex-automata	8m 58s
test (stable, ubuntu-latest, stable)	3m 56s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 44s
miri-regex-automata	10m 12s
msrv	2m 26s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 31s
rustfmt	13s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	8m 46s
testfull-regex	3m 46s
testfull-regex-capi	27s
test (beta, ubuntu-latest, beta)	3m 51s
testfull-regex-syntax	2m 7s
test (nightly, ubuntu-latest, nightly)	3m 0s
test (macos, macos-latest, stable)	2m 28s
test (win-msvc, windows-latest, stable)	5m 55s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 28s
	1h 9m 48s

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