Add support for the nightly unstable allocator API #521

Job	Run time
minrust	2m 22s
rustfmt	7s
miri	8m 3s
loom	2m 31s
nightly	3m 36s
stable (ubuntu-latest)	19s
tsan	2m 41s
stable (macos-latest)	33s
stable (windows-latest)	31s
cross (i686-unknown-linux-gnu)	34s
cross (armv7-unknown-linux-gnueabihf)	31s
cross (powerpc-unknown-linux-gnu)	28s
cross (powerpc64-unknown-linux-gnu)	35s
cross (wasm32-unknown-unknown)	10s
Publish Documentation	0s
	23m 1s

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