use rational_kernel_polynomial() in isogenies_prime_degree_general()

[yyyyx4]

...for a pretty huge speedup in computing ℓ‑isogeny neighbours in some (really: most!) cases.

Example:

sage: E = EllipticCurve(GF(419), [1, 1])
sage: for l in [37, 43, 53, 61, 67, 73, 79, 83, 89, 97]:
....:     print(l, '|', timeit(f'E.isogenies_prime_degree({l})'))

Timings from 10.10.beta4:

37 | 5 loops, best of 3: 329 ms per loop
43 | 5 loops, best of 3: 398 ms per loop
53 | 5 loops, best of 3: 3.08 s per loop
61 | 5 loops, best of 3: 6.11 s per loop
67 | 5 loops, best of 3: 33.2 s per loop
73 | 5 loops, best of 3: 37 s per loop
79 | 5 loops, best of 3: 3.92 s per loop
83 | 5 loops, best of 3: 3.15 s per loop
89 | 5 loops, best of 3: 33.7 s per loop
97 | 5 loops, best of 3: 24.3 s per loop

Timings from this branch:

37 | 5 loops, best of 3: 79.5 ms per loop
43 | 5 loops, best of 3: 103 ms per loop
53 | 5 loops, best of 3: 44.3 ms per loop
61 | 5 loops, best of 3: 58.9 ms per loop
67 | 5 loops, best of 3: 71.4 ms per loop
73 | 5 loops, best of 3: 85.6 ms per loop
79 | 5 loops, best of 3: 271 ms per loop
83 | 5 loops, best of 3: 292 ms per loop
89 | 5 loops, best of 3: 129 ms per loop
97 | 5 loops, best of 3: 157 ms per loop

Depends on: #42436. (This branch is based on that one; only the most recent commit is new.)

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