cargo: set 'default-features = false' for memchr and aho-corasick #2083

Job	Run time
miri-regex-automata	7m 23s
msrv	2m 6s
rustfmt	8s
test (stable, ubuntu-latest, stable)	3m 48s
testfull-regex	3m 48s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 46s
testfull-regex-automata	8m 44s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 41s
testfull-regex-capi	22s
testfull-regex-syntax	2m 3s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 19s
test (beta, ubuntu-latest, beta)	4m 0s
test (nightly, ubuntu-latest, nightly)	3m 49s
test (macos, macos-latest, stable)	9m 9s
test (win-msvc, windows-latest, stable)	7m 6s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 41s
	1h 15m 53s

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