Trac #16521: Add is_perfect() for fields

Currently there is no way to find out whether a field is perfect, i.e.,
whether its characteristic is zero or whether every element has a p-th
root. This is sometimes necessary to guarantee the correctness of
certain algorithms, e.g., #16518.

URL: http://trac.sagemath.org/16521
Reported by: saraedum
Ticket author(s): Julian Rueth
Reviewer(s): Martin Albrecht