gh-35370: Faster Kohel isogenies without bivariate polynomials
### :books: Description
This patch accelerates computation of Kohel formulas by replacing
internal bivariate polynomials k[x,y] by a tower of polynomial rings
k[x][y].
Because the y-coordinate of isogenies are always defined by a polynomial
of y-degree 1, this is equivalent to working with a pair of univariate
polynomials, which often have efficient representations especially over
finite fields.
The public API still exposes bivariate rational functions and is not
modified.
The resulting representation is several times faster.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
This change is self-contained but is meant to be combined with 2 other
changes:
- (to be published) faster `__call__` for NTL ZZ_pX polynomials
- #35358 : provides additional performance (independent patch)
URL: https://github.com/sagemath/sage/pull/35370
Reported by: Rémy Oudompheng
Reviewer(s): Lorenz Panny
