Add `BytesMut::try_reserve()` #729

Job	Run time
loom	32s
minrust	1m 18s
cross (i686-unknown-linux-gnu)	29s
miri	12m 7s
cross (armv7-unknown-linux-gnueabihf)	25s
nightly	54s
cross (powerpc-unknown-linux-gnu)	29s
rustfmt	3s
cross (powerpc64-unknown-linux-gnu)	34s
stable (ubuntu-latest)	1m 8s
tsan	1m 16s
stable (macos-latest)	1m 5s
stable (windows-latest)	2m 9s
cross (wasm32-unknown-unknown)	6s
Publish Documentation	0s
	22m 35s

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