Make `BytesMut::try_unsplit` method public #766

Job	Run time
loom	19s
miri	12m 3s
minrust	1m 55s
cross (i686-unknown-linux-gnu)	28s
nightly	57s
cross (armv7-unknown-linux-gnueabihf)	36s
rustfmt	5s
cross (powerpc-unknown-linux-gnu)	26s
stable (ubuntu-latest)	1m 27s
cross (powerpc64-unknown-linux-gnu)	32s
tsan	1m 22s
stable (macos-latest)	1m 8s
cross (wasm32-unknown-unknown)	20s
stable (windows-latest)	2m 49s
Publish Documentation	15s
	24m 42s

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