Change .list() to return an immutable object in enumerated sets.

[hivert]

Currently, in enumerated sets, the method .list() returns a standard mutable python list. This is unfortunate because, even if we use caching, we need a new copy of the list to make sure that the user doesn't change the cached list. This copy is made, even if the user don't change the returned list. The good way of resolving this problem must be discussed further but I nevertheless create the ticket for reference.

[vbraun]

+1 from me ;-)

Just to repeat what I said on sage-devel: List can be understood as "Python list", but also as as the English verb for "series of records". In the latter sense it is perfectly fine to return a Python tuple. But if you really can't live with the cognitive dissonance you can call it enumerate() with an alias list().