[Eva] Rewrites part of values_request.ml.

The evaluation [v_before] is now mandatory in the getValues output.
The getValues request now returns a record for each evaluation, containing:
- the resulting value as a string;
- the alarms emitted by the evaluation;
- the variables pointed to by the value, if any. This list is also computed
  when the result is an offsetmap (for struct and arrays).

Removes request [getPointedLvalues].