`serde` support?

[DenizUgur]

I’m happy to submit a PR for this, but I wanted to ask how we should handle serializing map in unsync version and shards in sync. If you’ve already started working on this, I’d be glad to help bring it to the finish line.

[arthurprs]

I think built in serde support is unlikely given iterators are exposed. Especially in the sync (sharded) cache it can be non-obvious, as the items will get re-arranged between shards (different Hasher seed) and that cause some items to be evicted during deserialization.