[WIP] Refactor visiting strategy

[zertosh]

This PR does away with the custom traverser, and instead adds correct node visitors to get accurate scopes (and references/definitions) for flow types. It's written for easy upstreaming into eslint-scope (see #438) if that becomes a thing. How? New node type visitors are added (look for //new), and only node type visitors were overwritten (look for //override) instead of visit* functions. There's some copy/paste from escope in the overwritten methods to clearly show where the new flow type code must go (look for <custom></custom>).

TODO:

• Fix declare (Correct scoping, especially with declare module).
• More scope tests.

[hzoo]

fyi eslint pr is eslint/eslint#8280

[kaicataldo]

Is this still something we want to do?

[hzoo]

and @zertosh if you have time to work on this again, I think a refactor PR sounds great!