Optionally use __slots__ for payload members

[misakwa]

Fixes #198

• Add TSPayload for slot based structs
• Add new optional use_slots parameter to allow generating code with slots
• Cache slot based objects slightly differently to avoid cache poisoning

[misakwa]

There are still a few questions and some cleanup to be done, but I think this should get the conversation started. /cc @dan-blanchard

[hit9]

Feature accepted. Update this issue later.

[misakwa]

I have updated the implementation to support all types of objects as well as the recursive case. The service args and results are now created with slots as well.

[misakwa]

This is now ready for review.