Currently, point addition for elliptic curves is only defined in EllipticCurvePoint_field and its children. This patch implements point arithmetic for EllipticCurvePoint, while also keeping the specialized (faster) implementation for fields. This is useful for symbolic computations, such as the ones that show up in a basic version of Schoof's algorithm (where we compute in a polynomial ring modulo the ideal generated by a division polynomial and the curve equation).

Another implication of this is that we can remove part of the "temporary" hack for elliptic-curve points over ℤ/n that was introduced in #1975.

Depends on #34417
Depends on #34463

CC: @JohnCremona @roed314 @edgarcosta @alexjbest

Component: elliptic curves

Author: Lorenz Panny

Branch/Commit: public/generalize_elliptic_curve_point_addition_to_rings @ 940f8a2

Issue created by migration from https://trac.sagemath.org/ticket/33228