[serapi] Parametric serialization support / JSON support

[ejgallego]

As discussed in chat rooms, we would like to provide a JSON interface to SerAPI; more generally, we would like to make the main code parametric w.r.t. serialization method so people could plug new serializers.

In general, this won't be an easy task. A few considerations:

• sercomp may be easier to handle, as discussed with @palmskog ; it should be easy to add a new option; however JSON support is not yet complete, for example I think plugins are missing.
• regarding SerAPI protocol, there are two main choices:
  • parameterize the code with a serializer parameter, for example by making it a functor
  • supporting several formats simultaneously, extending PpSer to take an argument.

It is not clear to me yet what should we do w.r.t. this last point.

[ejgallego]

Serlib does provide json serializers for 0.7.0, delaying this to 0.8.0 for now.

[cpitclaudel]

Hey Emilio! As a follow-up on https://github.com/janestreet/sexplib/issues/25, any chance to add JSON support to sertop?

[ejgallego]

Yes, it is pretty advanced, as a side effect of the direct python support. Unfortunately I found some problems with ppx_python / pythonlib so it got a bit delayed, but hopefully that goes into the next release.