Support structured labels

[buzden]

Currently with the model coverage mechanism we can provide structured resulting information:

Whatever.FieldList.FieldIsTyped covered partially
  - Here: covered
  - There: not covered

Whatever.FieldList.FieldList covered fully
  - Ext: covered
  - Nil: covered

Data.Fin.Fin covered fully
  - FS: covered
  - FZ: covered

To achieve this, we are using labelling mechanism, which now supports only plain Strings as labels.

We could

• support typed labels, so we can say that this is mentioning of a type or a constructor, or something else; open question: if set of this types should be open or preset;
• support structured labels, so we can say that e.g. a constructor label is a sublabel of a type label; using this, we could get such structured labelling information automatically; open question: currently, model coverage has a macro that calculates the full set of possible labels beforehand, but just addition of a support for structured labels won't give this ability, so this won't replace the current mechanism of model coverage; maybe, we should support additional labelling mechanism as "potential labels", i.e., a generator says "oh, by the way, I can in theory use the following (structured) labels"; will this impact runtime performance badly?

[buzden]

To think: a label type of "begin the subblock X", "end the subblock X"