Trac #15046: Symbolic elliptic integrals

This ticket is for making the elliptic integrals symbolic, including
arbitrary-precision numeric evaluation.

Defect because `elliptic_e(2.5,2.5).n()` throwing an ECL error is fixed
with it.

URL: http://trac.sagemath.org/15046
Reported by: eviatarbach
Ticket author(s): Eviatar Bach, Ralf Stephan
Reviewer(s): Karl-Dieter Crisman, Travis Scrimshaw

Author: Release Manager