extend ncwidth to automatically survey a terminal+font+libc

[dankamongmen]

Now that we've added code to ncwidth to get true length of glyphs using cursor location reports, we ought add a mode which sweeps through all of Unicode, and makes a note of which, if any, glyphs occupy more or less space than wcswidth() would lead one to believe.

I'm not certain what good this will do, but it would be interesting to see the results.

At that point, it might make sense to make ncwidth a tool we install, or perhaps fold it into notcurses-info. I'm thinking the latter might be the better idea.