Add zero-copy make_mut #641

Job	Run time
miri	43s
loom	1m 57s
minrust	1m 57s
cross (i686-unknown-linux-gnu)	43s
nightly	2m 58s
rustfmt	12s
cross (armv7-unknown-linux-gnueabihf)	38s
stable (ubuntu-latest)	1m 21s
cross (powerpc-unknown-linux-gnu)	34s
stable (macos-latest)	1m 11s
tsan	2m 19s
cross (powerpc64-unknown-linux-gnu)	35s
stable (windows-latest)	2m 10s
cross (wasm32-unknown-unknown)	13s
Publish Documentation	1m 45s
	19m 16s

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