Consider reducing the dependencies on JaneStreet libraries; especially, break-up ppx_jane

[lukstafi]

They put a load on the CI.

• Highest priority: ppx_jane pulls in a lot of packages.
• Medium priority: we don't use much from core.
• Low priority: get rid of base, i.e. all except sexplib0.

[lukstafi]

@nilsbecker: do you have any specific opinions on this?
Decision for now: keeping Base; Core seems trivial to remove; ppx_jane splitting will make it clearer what we're using.

[nilsbecker]

I don't have anything profound. It would be desirable if a user of the library would be free in choosing to use Base or not, in application code.