var_names/obs_names as fixed-size types

[rcannood]

At some point, the var_names and obs_names could be something other than strings in Python (See scverse/anndata#777). R data frames can only have characters as rownames, so we'll need to find a workaround.

Originally, we decided to not use the rownames of obs and var in anticipation for the aforementioned feature, but I reverted this in #171 to make the code cleaner / easier to read for now.

[rcannood]

As noted by @lazappi in #171 (comment):

I would probably make a special man page and/or vignette for things we know are different from Python.

I totally agree! If #777 is merged, we should add this to the list of known incompatibilities.