Add zero-copy make_mut #649

Job	Run time
minrust	2m 7s
loom	2m 17s
cross (i686-unknown-linux-gnu)	35s
miri	10m 57s
cross (armv7-unknown-linux-gnueabihf)	35s
nightly	2m 57s
cross (powerpc-unknown-linux-gnu)	37s
rustfmt	12s
cross (powerpc64-unknown-linux-gnu)	44s
stable (ubuntu-latest)	1m 19s
cross (wasm32-unknown-unknown)	12s
tsan	2m 19s
stable (macos-latest)	1m 6s
stable (windows-latest)	2m 39s
Publish Documentation	1m 44s
	30m 20s

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