Bring back `self.from_json` and `to_json` for CSP

[oreoshake]

Added in #163 and removed in the 3.x upgrade, it may be worth adding this back.

While there's no official spec representation, CSP clearly maps to a json object.

We should guarantee interop with anyone else who has implemented it

• Figure out who supports this
• Implement parity for lib X
• Implement parity for lib Y

Most of the complexity comes from parsing a policy and applying any "merge" conditions as defined in https://gist.github.com/jonathanKingston/5699b440f608960dc089. to_json is trival at worst.

/cc @jonathanKingston in case he can help me define that task list of libraries with which I should verify interop.

[oreoshake]

props to https://twitter.com/call_user_func/status/723353164703891457 for making me think about this again.

[oreoshake]

Until we have a use case...