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.
Note that other code-generating TTG-backends (namely python and
typescript) emitting ones already support this. This is just a matter
of extending similar logic to the c++ backend.
The `namespace` property is strictly used just by the cpp generator.
Move the code so the `--namespace` option is exclusive to it and removed
for all the other generators.
Merge all the command-line scripts for generating tuple-tree
implementations into a single `tuple-tree-generate.py` script. Move all
the other files a directory down.
Giacomo Vercesi