Kern commands accept braced size argument

In LaTeX, kern commands, such as `\kern`, `\mkern`, `\hskip`, `\hspace`, and `\mskip`, don't accept braced size argument, i.e., don't allow `\kern{1mu}`.

I'm currently working on argument parsing, and it'd be better to separate unbraced and braced size argument parsing, but it'd be a breaking change.