[with patch, positive review] fix lyndon and standard factorization of words

[saliola]

The last 1 in the word disappeared:

sage: Word([1,2,1,3,1,2,1]).lyndon_factorization()
(1213.12)

Lyndon factorization of the empty word should work.

sage: Words('01')('').lyndon_factorization()
Traceback (most recent call last):
...
StopIteration

The standard factorization of 321 is 32.1.

sage: sage: Word([3,2,1]).standard_factorization()
(321.)

The standard factorization of the empty word should return the empty word, and not two copies of the empty word.

sage: Words('123')('').standard_factorization()
(.)

[saliola]

based on sage-4.1.1.alpha0

[saliola]

This new implementation is correct and also faster.

sage: sage: Word([1,2,1,3,1,2,1]).lyndon_factorization()
(1213, 12, 1)
sage: sage: Words('01')('').lyndon_factorization()
()
sage: sage: sage: Word([3,2,1]).standard_factorization()
(32, 1)
sage: sage: Words('123')('').standard_factorization()
()

I also changed the repr of the (word) Factorization class to use ',' instead of '.' because otherwise the period is confusing if you factor a long word:

sage: WordOptions(truncate_length=10)
sage: tm = words.ThueMorseWord()
sage: tm[:100].lyndon_factorization()
(011, 01, 0011, 00101101, 0010110011..., 0010110011..., 0010110011..., 0010110011, 0)