Rename `DenseMatrix::columns_effective` to `DenseMatrix::stride`

lightmotif-py/lightmotif/lib.rs

lightmotif/src/dense.rs

lightmotif/src/pli/platform/avx2.rs

lightmotif/src/pli/platform/neon.rs

lightmotif/src/pli/platform/sse2.rs