facade/v0 package

Package facade is the permanent entry point of the "self-registering implementation" upgrade pattern (pattern E of the exploration; see ../../README.md).

The path callers import never changes and holds no business logic: it holds an interface value. A new implementation realm takes over simply by being deployed, because it registers itself from its own init. Nobody has to send a transaction to switch, which is the pattern's whole appeal and also its whole risk: whoever can deploy under the guarded prefix can take the realm.