Type codebase with flow

[aweary]

I have a feeling some types would make maintaining/refactoring freactal a lot easier. I'm looking at implementing #51 and it would be helpful if everything was well typed first.

cc @ryan-roemer

[aweary]

@divmain would you be OK with this?

[ryan-roemer]

@divmain -- Any last objections to applying flow to freactal? We'd ideally like to get this going soon-ish unless you have objections.

[divmain]

While I'm a fan of static typing in general, I'm on the fence with this proposal. The freactal code-base is small enough where typing is ideally unnecessary. I think exposing types for library consumers would be fantastic, but I view that as fundamentally separate.

@aweary, which parts of the code-base lead you to feel types would ease maintenance?