Prototype for conversion to CSTParser.EXPR

[c42f]

This demonstrates the basic approach working, but various things don't
work here yet due to at least two things:

• Mismatch between the way the two packages tokenize the input, eg, for
  the delimiters in strings.
• Missing "trivia nodes" in JuliaSyntax (eg, a brackets node). These
  should probably be added.

[c42f]

@pfitzseb here's an initial prototype of the CSTParser.EXPR conversion.

Currently it's still in the prototypes directory which I think is fine while experimenting.

However to make this usable but also avoid a CSTParser dependency we could either

• Put this in a separate compatibility package.
• Duplicate the CSTParser.EXPR data structure, in a JuliaSyntax.CSTParser compat module. But we'd also need to duplicate some CSTParser predicates and potentially the iteration code which could add up to a lot of code.

[davidanthoff]

Another option would be that CSTParser.jl takes a dependency on JuliaSyntax.jl, and just starts to use the parser her under the hood? We might even be able to keep the complete CSTParser API unchanged and non-breaking?

[c42f]

Sure! If everyone maintaining CSTParser is happy to do that, I think that's an excellent solution!

[sjkelly]

I suppose this would be even cleaner if CSTParser.jl incorporated this code as a package extension?

[davidanthoff]

I think it would still be cleaner to go with my idea from above. We have had serious difficulty maintaining even just one parser in CSTParser due to a lack of maintainer time, so if we were to go with a package extension story, we would effectively have to maintain two parallel things at the same time, which would presumably require even more time. Plus, I don't really see any benefit of that?