gh-37143: special subs for fractions
We create a specialised method `subs` for `FractionFieldElements`, for
better performance. In particular, the generic `Element.subs` method
insists on replacing all gens, which is a problem in polynomial rings
with many variables, such as (potentially) the `InfinitePolynomialRing`.
Fixes #37122.
URL: https://github.com/sagemath/sage/pull/37143
Reported by: Martin Rubey
Reviewer(s): Martin Rubey, Travis Scrimshaw
