* Added alternate preprocessor implementation "Preprocessor_Hash" which uses plain...

* Added alternate preprocessor implementation "Preprocessor_Hash" which uses plain PHP hashtables for the underlying data structure. This removes the DOM dependence, avoids XML charset problems, and avoids the lack of memory accounting in DOM. It will hopefully serve as a better model for alternate language implementations than the DOM version. I haven't measured the preformance yet, but it should be comparable to Preprocessor_DOM. If the performance is good, this might be a better default than Preprocessor_DOM.
* Verified Preprocessor_Hash against Preprocessor_DOM with a differential fuzz test.
* Improved the utility of Parser_DiffTest by synchronising strip markers and allowing for loose object comparison.
* Abstracted PPDStack to make it usable in Preprocessor_Hash.