Rework the python mixin infrastructure for the model. Mixins are now
specified explicitly instead of using the `ast` module. The mixins are
loaded relative to the generated python file, this allows to have two
models: `revng.model` and `revng.project.model`. The latter is augmented
via mixins to have project-specific facilities such as `get_artifact`
and artifact accessors.
When producing artifacts, if the rank is 0 the only element should be
extracted from the produced dict. Fix a wrong assertion that checked
that the returned dict has 0 keys instead of 1.
Add a python interface (`revng.profile`) for interacting with the rev.ng
infrastructure as a whole; either through the CLI (`CLIProject`) or the
GraphQL API (`DaemonProject`).
Giacomo Vercesi