Increase pool stacks to 32 #2496

Job	Run time
miri-regex-automata	14m 8s
msrv	1m 57s
test (stable, ubuntu-latest, stable)	3m 33s
rustfmt	8s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 54s
testfull-regex	3m 30s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 30s
testfull-regex-automata	9m 4s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 36s
testfull-regex-capi	23s
testfull-regex-syntax	2m 7s
test (beta, ubuntu-latest, beta)	3m 39s
test (nightly, ubuntu-latest, nightly)	2m 59s
test (macos, macos-latest, stable)	2m 17s
test (win-msvc, windows-latest, stable)	5m 38s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 21s
	1h 12m 44s

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