Add LRU cache

[connorjward]

We should introduce an LRU cache object/function to prevent unbounded memory usage by our in-memory caches. We will need to be careful about including the communicator in the cache key somehow so we never evict items from some ranks but not others.

Related issue #693.
Related discussion firedrakeproject/firedrake#2865

[wence-]

Reminder, that functools.lru_cache exists

[connorjward]

It does. But I think we need to be careful about communicators. I think that we need basically to have an LRU cache per communicator. Otherwise eviction won't be collective.

[wence-]

Natural thing to do is to hook the cache on the communicator as an MPI attribute then.

[connorjward]

Good suggestion, thanks! I still need to decide on the right API. I would quite like to implement this in tandem with my idea for a "cache manager" (#693).

[wence-]

Good suggestion, thanks!

Remember to do so on the inner "PyOP2" comm.

[connorjward]

Closing as solved by #724