Make \Rule produce mspace rather than mpadded so that negative values work

Currently, \Rule produces and <mpadded> element, but that means that negative values for height and depth are not processed, since the spec is explicit about the fact that you can't have negative height or depth on <mpadded>. This is not the case for <mspace>, and the HTML-CSS, CommonHTML, and SVG output all handle negative values for <mspace> height and depth.