Allow reclaiming the current allocation #654

Job	Run time
loom	2m 16s
minrust	2m 14s
cross (i686-unknown-linux-gnu)	39s
miri	4m 50s
cross (armv7-unknown-linux-gnueabihf)	36s
nightly	2m 12s
cross (powerpc-unknown-linux-gnu)	34s
rustfmt	12s
cross (powerpc64-unknown-linux-gnu)	35s
stable (ubuntu-latest)	22s
tsan	2m 23s
cross (wasm32-unknown-unknown)	14s
stable (macos-latest)	20s
stable (windows-latest)	30s
Publish Documentation	0s
	17m 57s

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