Implement FIFO and LRU caches through a Map memory limit

I would like to allow a max_bytesize to be set for a Concurrent::Map, which would stop the combination of the map and its entries from getting any bigger by automatically deleting entries when a new one is added. Deletion of the oldest entry is easy through Hash#shift. New entries would automatically make a FIFO cache, while an LRU cache would need each accessed entry to be deleted and re-added.

Is this something worth working up a PR for? If so, does anyone have any suggestions on what to do and what not to do?

Start with the Concurrent::Map entry point and review the issue discussion for a decision on scope. Clarify whether max_bytesize should support FIFO, LRU, or both, and define how size and access behavior are measured before implementation; the issue does not yet specify complete done criteria.