Document some commented-out code.
Remove more padding spaces from the AST.

The AST should have the raw function name, so that we can easily write
visitors that look for specific function names, etc.  Any necessary
space padding should be added in the render stage.
Add `--debug` option to CLI to get stack traces on errors.
Fix \begin{...} ... \end{...} environment parsing.
Expose/document low-level API and visitor helpers.