add `Bytes::is_unique` #502

Job	Run time
minrust	1m 21s
cross (i686-unknown-linux-gnu)	1m 14s
miri	8m 30s
loom	1m 25s
cross (armv7-unknown-linux-gnueabihf)	1m 9s
cross (powerpc-unknown-linux-gnu)	1m 22s
nightly	2m 27s
rustfmt	15s
cross (powerpc64-unknown-linux-gnu)	1m 27s
stable (ubuntu-latest)	1m 33s
cross (wasm32-unknown-unknown)	25s
stable (macos-latest)	1m 53s
tsan	1m 47s
stable (windows-latest)	2m 7s
Publish Documentation	1m 7s
	28m 2s

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