Create a IntegerSet class

For use in FiniteSubset and in intervals plugins. Must have a limit keyword argument that default to None which is the maximum integer value that these objects will contains.

• do we precise that the constructor must have Iterable of range?

• do we move the intervals method to a ranges method?

• do we move the from_intervals method to a from_ranges method?

• do we dump ranges using special mathematical notation?

  See https://en.wikipedia.org/wiki/Interval_(mathematics)#Integer_intervals).

  The notation [a .. b] is my favorite since it is used in Pascal.

Do we use 2 heaps (https://docs.python.org/3/library/heapq.html) for representing them?