Simpler parser ast

[Julow]

Hi!

As I explained in #355, Model.Comment is a bit hard to obtain and understand and too strict and lossy on the input.

Some of this issues could be solved by changing Model.Comment with a bit of work. But I think that in the future, this will make similar issues harder to solve as Odoc codebase and language change.

This PR simplifies Parser.Ast to make it closer to the concrete syntax and simpler by delaying some parsing to semantics.ml:

• References are represented as strings
• Raw_markup targets can be any string or none.
• Keep concrete syntax used for lists
• Keep whitespaces (Only when a Space element is generated)

This PR also removes some uses of Model types, as @lpw25 suggested, but not all yet.

This PR is supposed to follow #351 but if this approach is definitively rejected, it should be a good first step at improving Model.Comment.

What do you think?

Thanks!

[aantron]

Ok, having seen this change, it actually looks quite reasonable, and I think we can merge this.

Apart from the comments, there are also some CI failures on older versions of OCaml.

Let me know if you'd like #351 reopened, or submit a new PR for how you want to expose this AST.

I still would like to avoid committing to the stability of Model.Comment, Parser.Ast, or the parser, but we can reach out when making any breaking changes.

[Julow]

Thanks for your review !

I cleaned the code but did not manage to fix the CI with 4.08 yet.
Yes, I'd like to reopen #351 if possible, I just rebased the branch. The stability should not be an issue for us.

[aantron]

Thanks! The 4.08 failure is not due to this PR, and is tracked in #367. It seems GitHub won't let me reopen #351, because the underlying branch has been rebased. Would you mind opening a new PR, or pinging me if you move the branch back to the last commit in #351 when it was open?

[Julow]

Thanks for merging !
I changed the branch of #351.