Implement kernel_points and inverse_image for elliptic curve hom

[user202729]

Ideally I want to implement .kernel_gens() (and eventually .kernel()) but that requires a lot of framework (subgroup of E.abelian_group(), etc.)

Also it may be possible to use a more efficient algorithm for EllipticCurveHom_composite, but I haven't worked out the math yet. (if you're lucky then taking the inverse image over each successtive φ works, but otherwise…)

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