Use a PORO for field extension state instead of a Hash

[rmosolgo]

When you have a field extension that doesn't produce a memo or modify object or arguments and doesn't add any extras, this will remove the allocated Hash from that case. That's not possible, but I think using a PORO will still improve performance.

[rmosolgo]

Ahh -- I forgot about the case where def resolve returns without yielding. In that case, GraphQL-Ruby has no chance to "pick up" the extended arguments at the end of the extension chain. That's why it has this Hash in the first place, it's a place to store this state while the chain is running, even if it ends early. We have to have some kind of out-of-bounds state object here. Maybe it could be a PORO instead, but the win there is minimal at best.