Patch by MinuteElectron - https://bugzilla.wikimedia.org/attachment.cgi?id=4772 with fixes
Replaces an instance variable with a constant, since it doesn't change, which is safe to access when the function is called statically from an outside class, as some extensions do.
* 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.