Milestone: Feature parity with parsertoy-php

[xparq]

Almost there -- _SAVE (and possibly other extensions) in the old tests are not yet added etc.

(Not that there are "features", BTW... This can only check the syntax of a source, nothing else yet.)

[xparq]

Unsurprisingly, the roughly equivalent C++ code is immensely more complex, tedious, noisy, messy and rigid.
It's ~3 times as long, too.

And since this is an inherently exponential recursive backtracking algorithm, I don't think the performance "gain" here is a meaningful concept at all. :)