Simplify 2-d matrix types

[plafer]

There are currently multiple types representing 2 dimensional matrices: Vec<Vec<E>>, RowMatrix, ColMatrix, and maybe others. I suggest we only have 2: RowMajorMatrix and ColumnMajorMatrix.

• Each have an internal data buffer, similar to what today's RowMatrix has
• In each, you can read/write rows and columns; only one of which would be most cache efficient (e.g. RowMajorMatrix would have read/writing rows fastest).
• The entire codebase would use these - you would never see Vec<Vec<E>>. For example, EvaluationFragment::update_row() would simply delegate to the internal ColumnMajorMatrix as opposed to (re-)implementing that logic there.

The main benefits of this approach are:

• readability: I don't need to wonder if Vec<Vec<E>> stores columns or rows
• reusability: any custom logic across the codebase for Vec<Vec<E>> (such as this EvaluationFragment::update_row(), ColMatrix, RowMatrix) would all live in one place
• performance: these are more performant than Vec<Vec<E>> (cache locality)