Use CSV::parse with block

[buren]

CSV::parse with block will yield each row of the CSV file. If we use that method we could get rid of the extra rows iteration that we need we now do to build the row objects.

Though in order for that to happen we need to update the internal interface of Matrix.

[buren]

After some initial, albeit very limited, testing it seemed to my surprise that it wasn't faster at all, but slower. (I must have made some other simple change during that PoC that caused that.)