genericMatrix should be row major

It was pointed out during the workshop that the output of `genericMatrix` should be row major instead of column major because this makes no sense:
```m2
i1 : R = QQ[x_(1,1) .. x_(2,5)];

i2 : genericMatrix(R, 2, 5)

o2 = | x_(1,1) x_(1,3) x_(1,5) x_(2,2) x_(2,4) |
| x_(1,2) x_(1,4) x_(2,1) x_(2,3) x_(2,5) |

2 5
o2 : Matrix R <-- R
```

The issue identifies the genericMatrix entry point and shows its current column-major output for a 2x5 matrix. Start by locating genericMatrix and any existing tests, then compare their ordering with the example. Done means genericMatrix produces row-major output and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.