Rewrite asdl scripts in Rust

[Techcable]

Right now build is just a stub that calls out to python code.

This is rather unfortunate, since it requires the user have a python3 executable available on their system.

Even worse, the Python scripts have external dependencies (outside of the stdlib):

• click (See issue The python scripts require click #2)

There is an implicit advantage of this is of course that there are much fewer build-dependencies :)