Exposed interface Sink not usable

I am currently trying to implement a custom sink but the definition has an [unexported method](https://github.com/golang/groupcache/blob/master/sinks.go#L41-L42) which makes it impossible to implement a custom `Sink`. Is this intended, if not, will you accept a PR changing it?

My use-case for a custom Sink, currently I am caching 4MB chunks and would like to only read 500 bytes at a time at different offsets. But for each call, groupcache allocates full 4MB when calling `cloneBytes()`, I would like to allocate right amount to reduce allocs.

cc @bradfitz