Allow reclaiming the current allocation #601

Job	Run time
miri	10m 55s
loom	1m 34s
minrust	1m 21s
cross (i686-unknown-linux-gnu)	29s
cross (armv7-unknown-linux-gnueabihf)	30s
nightly	2m 28s
rustfmt	4s
stable (ubuntu-latest)	1m 20s
stable (macos-latest)	3m 30s
cross (powerpc-unknown-linux-gnu)	27s
tsan	1m 57s
stable (windows-latest)	2m 45s
cross (powerpc64-unknown-linux-gnu)	31s
cross (wasm32-unknown-unknown)	10s
Publish Documentation	1m 6s
	29m 7s

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