Test: JSON Roundtrip

It would be cool to have some quickcheck tests that:

1. Generate some behaviours
2. Write those behaviours to JSON
3. Parse the JSON into a behaviour
4. Assert equality of the parse behaviour to the original

To do this we would need to implement `FromJSON` instances in `RefinedAST`, which adds some complexity, but it probably required anyway if we want to allow external tools to programatically generate act.

Start by reading the RefinedAST definitions and the existing JSON parsing or serialization code. Add the FromJSON instances needed for behaviours, then create QuickCheck properties that generate behaviours, round-trip them through JSON, and assert equality with the originals.

Written by the indexing model from the issue text.