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.
This also unifies `get_default_value` helper across all the target
languages.
Before this, there were two different versions of it for python
and typescript while c++ side didn't have one at all.
After this commit, every non-key field is treated as if it was optional
while every key field (plus every auto-generated `Kind` field) - as if
it was required.
Add a class implementing the descriptor protocol to allow TypedList
fields to be assigned a `list` and automatically convert it to the
TypedList class.
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.
Ivan Krysak