Generate d.ts declarations

[alfonsogarciacaro]

This PR extends Fable AST to make it easier to add Flow type annotations when transforming it into Babel AST. These annotations can then be used by the babel-dts-generator plugin to generate d.ts declarations.

The declarations are far from perfect. These are some of the things missing:

• Generic arguments: everything is already laid down to add them, we only need to fill in the gaps.
• Class properties: a bit more of work, as class properties have to be added explicitly at least for unions and records.
• Interfaces: it also requires some work as Fable is ignoring abstract methods at the moment.
• Nested modules: I can foresee some pain here.

I'll try to implement at least the first two points before merging, but probably the other two will be postponed to another release.