gh-36299: Generic implementation of fitting ideal
and specific _fitting_ideal for MPolynomialRing
This implements a method `fitting_ideal` for generic matrices, that only
does a simple optimizations, and a more optimized version (named
`_fitting_ideal` ) for matrices over polynomial rings with multiple
variables.
Fixes #36298
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URL: https://github.com/sagemath/sage/pull/36299
Reported by: miguelmarco
Reviewer(s): Enrique Manuel Artal Bartolo, miguelmarco
