Add a way to tell if Bytes is unique

[zyxw59]

I would like to be able to tell if a Bytes object is the unique reference to the underlying data. The usecase is a cache, where I want to be able to evict an object from the cache only if it is not used elsewhere — otherwise, removing it from the cache would not free up any memory, and if the object was requested again later, it would have to be fetched (in my case, thru a network request), and duplicated in memory if the original copy is still around.

[Darksonn]

Seems reasonable enough to me.

[NobodyXu]

@zyxw59 What about instances created using Bytes::from_static?

[zyxw59]

I think in that case it is probably correct to always say that it is not unique (since there always might be other copies)

[NobodyXu]

I think this can be easily implemented by adding another function to Vtable of Bytes.