Allow reclaiming the current allocation #602

Job	Run time
minrust	1m 22s
miri	10m 26s
cross (i686-unknown-linux-gnu)	35s
loom	1m 35s
cross (armv7-unknown-linux-gnueabihf)	33s
nightly	2m 31s
rustfmt	4s
cross (powerpc-unknown-linux-gnu)	35s
cross (powerpc64-unknown-linux-gnu)	32s
stable (ubuntu-latest)	1m 29s
tsan	1m 44s
cross (wasm32-unknown-unknown)	5s
stable (macos-latest)	1m 43s
stable (windows-latest)	2m 0s
Publish Documentation	1m 15s
	26m 29s

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