Server mode

[pitag-ha]

A server mode for the driver would be quite useful for tools such as Merlin or Dune to improve performance in the interaction with the PPX driver on a systems level. The main advantages would be:

• Avoid spawning a new process for every PPX invocation.
• No need to pass the ASTs via disk anymore.
• Simple cancelability.

A few things to keep in mind if we implement this:

• There are some PPXs relying on global state. So we'll need to make sure we clear the state between two server invocations.
• Implement this in a Merlin-backward-compatible way.