Add mechanism for pinning `flatc` to a specific version

[rerpha]

As per discussion with @Tom-Willemsen and @DanNixon (a while ago now!)

We should pin the flatc version somehow to avoid generated bindings changing on new versions and/or when adding schemas.

This could be done by renaming generate_rust_bindings.py to generate_bindings.py, which goes and pulls a pinned binary of flatc from github, sticks it in the current working directory, then calls flatc --python and flatc --rust with the appropriate arguments

[Tom-Willemsen]

CLF mechanism for generating their high-level bindings may be inspiration (see dev.py).