Always generate row names

[MichaelChirico]

Closes #175

This PR is currently incomplete -- checking it in as a first step towards a solution.

Basically I'm suggesting {repr} override the default dimension printing (the style of which is more-or-less unchanged in base R for "forever": https://github.com/r-devel/r-svn/blame/main/src/main/printarray.c) in favor of defaulting to just applying the column/row number.

• If that looks good to you, there's a little bit of clean-up we can do where other parts in the code refer to rownames() possibly being empty that are now redundant.
• Otherwise, let's discuss alternative approaches.