Plain fonts

[brucemiller]

This PR gets most of the low-level font related commands better cooperating, in particular control sequences defined by \font are now represented by a FontDef which then will change to the font, and is also recognized by \the and \meaning, as well as the accessors \fontdimen, etc. These commands are also stored in \textfont,\scriptfont,\scriptscriptfont for use with \fam and the \mathcode mechanism to choose fonts for math tokens in a particularly interesting way, which now mostly works. Consequently most of plain.tex mostly works.

This mostly cooperates with the higher-level LaTeX style of font selection currently used by LaTeXML (by specifying family, shape, etc, w/o explicitly naming fonts or font files); later work will need to integrate these better.