Pin asdl and vendor previous version in laplace.third_party

[kourbou]

This PR fixes pip installation on main by pinning the version of asdl to commit 011a942 and vendoring the previous version in laplace.third_party. I've copied over the LICENSE file from asdl but feel free to let me know how else I can give proper attribution. Maybe we should consider linking the original source in a doc-string in __init__.py.

[aleximmer]

Thanks for preparing this draft. If we go down this route, I would suggest removing all functionality that we do not need, for example fr.py and hessian.py and precondition.py could be such examples. Should be easy to check for dependencies automatically and keep what's required. Or is there some argument for keeping all old functionality as well?

[kourbou]

Yes, we can remove unused submodules. Test coverage for the old backend should be good enough for keeping only relevant code. I think the argument in favor of keeping all the code is if users of laplace-torch want to re-use other components of ASDL in a same project, but then it's better for them to migrate to the new backend anyways?

[wiseodd]

We decided to go with #225. Thanks a lot, nevertheless, @kourbou!

[aleximmer]

Closed due to #238, which integrates a pruned and formatted version of asdfghjkl. Thanks for the idea and draft @kourbou!