let solve delegate to roots

[rwst]

A natural convenience improvement is possible. At the moment:

sage: z = var('z')
sage: f =  1 - z - z^2 - z^3 - z^4 - z^5
sage: solve(f == 0,z)
[0 == z^5 + z^4 + z^3 + z^2 + z - 1]
sage: f.roots(ring=QQbar)
[(0.5086603916420041?, 1),
 (-1.011836827437571? - 0.6839585956421031?*I, 1),
 (-1.011836827437571? + 0.6839585956421031?*I, 1),
 (0.2575066316165687? - 1.118790314198966?*I, 1),
 (0.2575066316165687? + 1.118790314198966?*I, 1)]

Algebraists know how to use Sage's ring elements, so they are not relevant here. Calculus users are satisfied to get all roots in the most general ring, i.e., QQbar for degree >3. So, in the uni polynomial case this should be the default behaviour instead of Maxima which does nothing.

Component: symbolics

Issue created by migration from https://trac.sagemath.org/ticket/18900