Add support for Union types, including completion

[MetaMemoryT]

NOTE: these modifications should be reviewed in the context of google/tern-closure#34

Some Union type functionality requires tern-closure to be tested, (that's why some of the new tests reside in tern-closure) as the doctrine parser is required to parse union types from annotations.

[marijnh]

I am not convinced this is the right direction. We already have sets of types modeled as AVals with multiple types inside of them. A Type is currently a single, definite type. Adding another mechanism for expressing sets of types at that level seems confusing.

I'd be okay with extending the parser to recognize the | operator as a way to add multiple types to something, and showing small sets (say <4, after merging similar types) of types in that same way in Aval.toString (though I haven't really thought through all the implications of that, I think on the whole it would be a win for clarity).

[MetaMemoryT]

Thank you for your explanations, I am revising my code and investigating how I could use AVals to achieve the same functionality.

[RReverser]

Did this go anywhere in the end?

[MetaMemoryT]

I realized I need to get funding to spend anymore time on this.