This PR removes the dependency of the Javascript worker to the legacy
frontend in order to remove it completely. In order to avoid code
duplication as it was done with the legacy frontend, we reuse the
`Solving_loop` module in the worker. This requires some modifications:

- Add a new argument to the main function of `Solving_loop` to give
  different kind of sources instead of reading the file at the path
  `Options.get_file ()`. The worker used a raw string as input. I reused
  the type `Dolmen_loop.State.source`.

- Add a new argument to the main function of `Solving_loop` to pass the
  selector hook to the constructor of the environment of the SAT solver.
  This hook is not used by the worker to select instantiations but just to
  record them. I have no strong opinion to preserve this feature but I
  try to keep all the feature of the Javascript worker.

- Remove the unused `sat_env` argument of the constructor `init_env` in
  `Frontend`. This removal is mostly motivated by the fact the semantic
  of `init_env = Some env` + `selector_inst = Some f` is not clear at
  all.

- Monomophize the `status` type in `Frontend`. This status is only used
  for printting purposes but we kept the SAT environment in some of its
  constructors. Unfortunately, the type of the SAT can change at the
  very beginning of the main function of `Solving_loop`, which means it
  is not easy to add a new argument for the `hook_status`. This hook is
  required to print the result of AE in the worker.

We do not need `In_channel.input_all` too ;)