concatenation of words should preserve the data type represention when possible

[slabbe]

This concerns word represented by str, tuple and list.

1. We want the representation of a concatenation of two words u and v to be preserved when u and v are of the same data type.

ACTUAL BEHAVIOR:

sage: u = Word(range(10))
sage: type(u)
<class 'sage.combinat.words.word.FiniteWord_list'>
sage: type(u*u)
<class 'sage.combinat.words.word.FiniteWord_callable_with_caching'>

sage: v = Word('asdgadsf')
sage: type(v)
<class 'sage.combinat.words.word.FiniteWord_str'>
sage: type(v*v)
<class 'sage.combinat.words.word.FiniteWord_callable_with_caching'>

sage: v = Word((2,3,5,21,34,6))
sage: type(v)
<class 'sage.combinat.words.word.FiniteWord_tuple'>
sage: type(v*v)
<class 'sage.combinat.words.word.FiniteWord_callable_with_caching'>

2. Moreover, the empty word could be ignored for concatenation :

ACTUAL BEHAVIOR:

sage: type(Word([]) * Word('asdga'))
<class 'sage.combinat.words.word.FiniteWord_callable_with_caching'>

INTENDED BEHAVIOR:

sage: type(Word([]) * Word('asdga'))
<class 'sage.combinat.words.word.FiniteWord_str'>

[slabbe]

Forget about this patch!

[slabbe]

Depends on #7619.

[abmasse]

Tested on sage-4.3.3 after having applied ticket #7619. All tests passed, looking at the documentation with browse_sage_doc reveals nothing to be corrected. The improvement seems very reasonable and the code looks fine. Positive review !