(string-rectangle-line): After inserting string, insert the same
width of whitespace that we deleted earlier.
(operate-on-rectangle): Operate even if width is 0.
(open-rectangle-line): Clean up.
argument instead of implementing it ourselves.
(rectangle-coerce-tab): No longer used; deleted.
* rect.el (open-rectangle): Leave point at the upper corner of the
rectangle just opened, in imitation of open-line. Fix doc typo.
(open-rectangle-line): Use skip-chars-backward's LIM argument to
leave spaces before the line segment's beginning undisturbed.
* rect.el: Provide 'rect.
