compute minimal polynomial of an isogeny & push such polynomials through other isogenies

[yyyyx4]

Back in #37125 we implemented Algorithms 3 and 4 from Deuring for the People. In this patch we add Algorithm 5, as well as a simple convenience method to compute the minimal polynomial of an isogeny.

This functionality is very convenient for computing chains of isogenies whose kernel points may live in extension fields — without explicitly constructing and working in those extension fields.

[yyyyx4]

I suppose this should have a specialized implementation in EllipticCurveHom_composite, so that we do the pushing step by step rather than constructing the rational map for the entire chain. Is it worth doing this now? Edit: Done.

[GiacomoPope]

LGTM, there's a FIXME, but I can't think of something obvious to replace this.