Groupie: Add support for diffing whole contents of the adapter

[lisawray]

The initial version of Groupie was designed before DiffUtil ever came on the scene. While creating and mutating individual groups can be really useful as an abstraction in your code, it can also be really useful to have a one-way data flow which produces immutable list contents. We should support both ways (shouldn't be difficult)

[lisawray]

This is in a new build, 2.1.0-alpha3.