Subview optimizations, round 4

This one is just code cleanups. (Like the others the diff won't make sense until the previous MR is merged.)

• Rename copy_array() to copy_mat() for consistency of terminology.
• Rename eop_array() to eop_mat() for the same reason.
• Reorganize and clean up some implementations in the backend directories.
• Use copy_mat() in a couple places where eop_scalar() was instead being used.