add a Unique function

[marten-seemann]

This is the function we optimized in libp2p/go-libp2p#2395. The plan is to remove this function from go-libp2p.

[MarcoPolo]

I think there should be a Unique and a UniqueSorted. UniqueSorted being the version that works if the list is already sorted.

[marten-seemann]

I think there should be a Unique and a UniqueSorted. UniqueSorted being the version that works if the list is already sorted.

Updated to Unique. We can add a UniqueSorted later, if the need arises.

[sukunrt]

Unique is nice.