Make hashString64Bit and friends public

[chrisbanes]

There are times when I might need to grab the model from the adapter using a model's ID. If I use id(String) then there there's no nice way to create the resulting long ID without the model.

I could do new Model().id("string").id() but that's wasteful.

[elihart]

Sure, sounds reasonable. Interested in putting up a PR?

[chrisbanes]

Sure, preference on class name? IdUtils?

[elihart]

That sounds great, and feel free to do it in Kotlin