compute matrix kernels modulo composites

[yyyyx4]

Currently, Sage cannot compute kernels of matrices over Zmod(n) for composite n.

This patch adds a call path from Matrix.right_kernel_matrix() to PARI's matkermod() function for matrices defined over IntegerModRings with composite modulus.

Also, for symmetry we add a .left_kernel_matrix() method that simply calls .transpose() and defers to .right_kernel_matrix().

Component: linear algebra

Author: Lorenz Panny

Issue created by migration from https://trac.sagemath.org/ticket/34862

[sagetrac-git]

Changed commit from 8ed83fa to a005c96

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

a005c96

fix doctests

[mkoeppe]

Removed branch from the issue description because it has been replaced by PR #34968