MapRef

This is a total map from K to Ref[F, V].

It is conceptually similar to a Ref[F, Map[K, V]], but with better ergonomics when working on a per key basis. Note, however, that it does not support atomic updates to multiple keys.

Additionally, some implementations also provide less contention: since all operations are performed on individual key-value pairs, the pairs can be sharded by key. Thus, multiple concurrent updates may be executed independently to each other, as long as their keys belong to different shards.