v0.3.0: maybe-final client API

This release contains a hopefully-final version of the client-application API, which is entirely Entry-based (with no use of credentials). Please read the design docs on the Keyring Wiki, especially the in-depth doc about the Keyring Core. There is info about how client code will need to change in this module's README.

The credential-store API is still not finalized. That comes next, and with it some credential stores.

Feedback on the API is very welcome!