add `Bytes::is_unique` #519

Job	Run time
loom	2m 29s
miri	7m 58s
minrust	2m 17s
cross (i686-unknown-linux-gnu)	27s
cross (armv7-unknown-linux-gnueabihf)	29s
nightly	3m 23s
rustfmt	3s
cross (powerpc-unknown-linux-gnu)	31s
stable (ubuntu-latest)	1m 9s
cross (powerpc64-unknown-linux-gnu)	29s
stable (macos-latest)	1m 38s
cross (wasm32-unknown-unknown)	11s
tsan	3m 3s
stable (windows-latest)	2m 50s
Publish Documentation	2m 12s
	29m 9s

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