Add json mapper for pp_ast

[pedrobslisboa]

Description

Add json mapper on pp_ast to improve usage in tools such as AST Explorer.
The idea is to increment the @NathanReb PR #517

@NathanReb, your opinion on how to structure it better is very welcome.
@jchavarri @davesnx thank you for the help.

How

To use it, we just need to pass --json to ppxlib-pp-ast. It works with all other flags, such as --show-attrs and --show-loc.

How to test

• In this branch build the project
• Create a test.ml on the root and add some content
• Run ppxlib-pp-ast test.ml and check the result
• Run ppxlib-pp-ast --json test.ml and check the result
• Use others flags to see the result with --json

[patricoferris]

Just a quick comment from me, I don't think we would want to add yojson as a dependency of ppxlib for this feature (I might be mistaken).

The implementation uses the lifter to lift values to JSON values (not actually print them, like pp_simple_val does). So I wonder if this couldn't be its own little tool that depends on ppxlib?

One workaround if this were to be merged here would be to add yojson as a dependency for ppxlib-pp-ast and then ppxlib could just return the polymorphic variants that happen to correspond to the Yojson.Safe.t variants?

[NathanReb]

Yes, as Patrick said, ppxlib should not depend on external libraries as it otherwise prevents them from being able to depend on ppxlib.

The Yojson dependent parts should go into ppxlib-pp-ast or just a seperate tool shipped with ppxlib-tools.

We could add a json lifter to ppxlib but given it's fairly easy to write one this could be done directly in ppxlib-tools.

Did you try to load the resulting JSON into AST explorer? It would be nice to have a working usecase for this before we merge this feature!