let-unimi/liblet: Preparing for LR lectures

- Modifications in the display module:
    - removed StateTransitionGraph.from_lr method,
    - added large_label parameter to StateTransitionGraph.from_automaton method,
    - added compact parameter to ProductionGraph.
    - added sep parameter to dod2table method,
    - added sort parameter to dod2table method (to control sorting of row/column headers),
    - methods cyk2table and prods2table generate a bordered (non wide) table,
    - added warn method (to display messages on sys.stderr),
    - added ff2table method (to display FIRST/FOLLOW table),
    - added dict2table method (to display a dict as a table),
    - method dod2table encloses cell content in <pre>,
    - method dod2table uses OrderedDict.fromkeys to make the list unique preserving column order,
    - reviewed table visualization methods.
- Modifications in the grammar module:
    - added Item.symbol_after_dot and Item.advance methods,
    - improved type checking for Transition,
    - Transition can be made from tuples of items,
    - renamed Grammar.context_free to Grammar.is_context_free (and added to __slots__).
- Modifications in the utils module:
    - added Stack.peek method (to inspect the top of the stack without pop).
- Improved documentation:
    - fixed the documentation of the modified methods and classes,
    - added documentation for grammar.Item,
    - regenerated examples (due to display module improvements),
    - uniformed right/left-hand side naming,
- Tests:
    - added tests for the Transition containing items,
    - added a test for the new Stack.peek method,
- Cleanup:
    - moved EarlyItem out of the library (it will not be used soon),