Introducing: result handlers

[cicdw]

Large portion of #389

This PR introduces a new tool in the core toolkit: ResultHandlers. A result handler is a configurable class for "serializing" and "deserializing" state results, where "serialize" means serialize plus place into remote storage, and "deserialize" means pull from remote storage and then deserialize. Note that currently this is only serializing state.result, not cached_inputs (that will be the next PR).

An instantiated result handler is (optionally) attached to a flow, and then passed down into client calls as needed.

In implementing this new feature, I also refactored the client into a full submodule.

Things left (not necessarily in this PR unless others want that):

• handle cached_inputs and cached_states
• handle expiration
• add default
• serialize these / package them up with the Flow for deployment