Make the C++ version of tree_multimap accept tree suffixes of the primary tree. Document and test this behavior.
Remove unnecessary locking in custom node registry; we hold the GIL already so there's no point to the additional locking.
Move jaxlib version test into jax/lib/__init__.py. Make jax/lib mirror the structure of jaxlib; e.g., xla_client is now available as jax.lib.xla_client.