Allow reclaiming the current allocation #667

Job	Run time
loom	1m 19s
minrust	1m 14s
miri	11m 8s
cross (i686-unknown-linux-gnu)	29s
nightly	2m 31s
cross (armv7-unknown-linux-gnueabihf)	26s
rustfmt	3s
cross (powerpc-unknown-linux-gnu)	26s
cross (powerpc64-unknown-linux-gnu)	34s
stable (ubuntu-latest)	1m 19s
tsan	1m 43s
cross (wasm32-unknown-unknown)	7s
stable (macos-latest)	1m 17s
stable (windows-latest)	3m 25s
Publish Documentation	1m 5s
	27m 6s

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