apply Maxima's simpsum in sum() and simplify*()

[rwst]

https://groups.google.com/forum/?hl=en#!topic/sage-devel/0deu-ScIpgA

sage: sum(2^(-x)+3^(-x), x, 1, oo)
sum(3^(-x)*(3^x + 2^x)*2^(-x), x, 1, +Infinity)
sage: sum(2^(-x)+3^(-x), x, 1, oo).simplify_full()
sum((3^x + 2^x)/(3^x*2^x), x, 1, +Infinity)
sage: maxima("sum(2^(-x)+3^(-x), x, 1, inf), simpsum;")
3/2

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/20520