Allow reclaiming the current allocation #598

Job	Run time
loom	1m 22s
minrust	1m 13s
miri	10m 21s
cross (i686-unknown-linux-gnu)	27s
cross (armv7-unknown-linux-gnueabihf)	27s
nightly	1m 21s
rustfmt	4s
cross (powerpc-unknown-linux-gnu)	30s
stable (ubuntu-latest)	24s
stable (macos-latest)	30s
tsan	1m 44s
stable (windows-latest)	28s
cross (powerpc64-unknown-linux-gnu)	29s
cross (wasm32-unknown-unknown)	6s
Publish Documentation	0s
	19m 26s

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