mathtext support for `\phantom`, `\llap`, `\rlap`#

mathtext gained support for the TeX macros \phantom, \llap, and \rlap. \phantom allows to occupy some space on the canvas as if some text was being rendered, without actually rendering that text, whereas \llap and \rlap allows to render some text on the canvas while pretending that it occupies no space. Altogether these macros allow some finer control of text alignments.