serde_v8

Serde support for (rusty_)v8

WIP: see denoland/deno#9540

TODO

• Experiment with KeyCache to optimize struct keys
• Experiment with external v8 strings
• Explore using json-stringifier.cc's fast-paths for arrays
• Improve tests to test parity with serde_json (should be mostly interchangeable)
• Consider a Payload type that's deserializable by itself (holds scope & value)
• Ensure we return errors instead of panicking on .unwrap()s