Allow reclaiming the current allocation #677

Job	Run time
miri	10m 52s
cross (i686-unknown-linux-gnu)	24s
loom	2m 30s
cross (armv7-unknown-linux-gnueabihf)	29s
minrust	2m 1s
cross (powerpc-unknown-linux-gnu)	26s
nightly	2m 59s
cross (powerpc64-unknown-linux-gnu)	25s
rustfmt	5s
cross (wasm32-unknown-unknown)	10s
stable (ubuntu-latest)	1m 25s
tsan	2m 34s
stable (macos-latest)	1m 1s
stable (windows-latest)	2m 4s
Publish Documentation	1m 38s
	29m 3s

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