Eventually Periodic word class

[sstarosta]

Implement a class for eventually periodic words.

The motivation is the following behavior.

sage: W = Words([0,1])
sage: w1 = W(lambda x: 1 if x%2 else 0)
sage: w1[0] == w1[2]
True
sage: w1[0] is w1[2]
False

While using periodic word class we can have

sage: w2 = W.periodic_word(period=[0,1])
sage: w2[0] == w2[2]
True
sage: w2[0] is w2[2]
True

It will be used in substitutive language #12227