feat(Parser): implement Parser

Add a simple parser implementation that supports only field reads.

@vicb @chirayuk @mhevery could one of you guys take a look?

WIP
I actually had the word Visitor there, but then I removed it. 
* `ProtoWatchGroup` does not care if this class uses the visitor pattern. 
* When working with `ProtoRecordCreator` it is clear that it is a visitor, because of `@IMPLEMENTS(AstVisitor)`.

I can add the Visitor suffix back if it is our convention.