Switch to Twig templates, remove PHP custom tags.

Replace the HTML/{placeholder} markup with templates that can be fed into the Twig engine.

This will probably allow some of the simpler callback tags (eg. [node], [url], etc.) to become template tags as well.

Finally (in what is a big feature kill): Remove PHP custom tags entirely. A lot of the simpler control-flow stuff can be accomplished with a template, and the rest can be done by creating a custom module. Using eval() is terribly out of fashion.

The upgrade path for this is going to be a bit iffy. Ordinary templates need to be converted (a simple pattern replacement), while PHP custom tags need to be completely disabled to avoid them getting exposed in the filter.