Postcard from linuxland.

Postcard from linuxland.
* Reduced stack depth by using an internal stack in expand(), and by having some common code paths (e.g. non-subst double-brace during PST) return objects which can be expanded in that internal stack instead of the PHP stack. This is friendly to xdebug but slightly slower than the original version. Also it probably helps robustness when you don't add 7 stack levels per pair of double braces.
* Profiling indicates that expand and PPD are now good targets for porting to C. Abstracted and refactored the relevant code to allow for a drop-in replacement. A factor of 2 reduction in average-case replaceVariables() time may be possible.
* Verified with preprocessorFuzzTest.php against r29950, updated to allow better PST tests.
* Made parserTests.php respect $wgParserConf
* LST and ParserFunctions need a simultaneous update with the core due to changed interfaces. DOM objects are now wrapped rather than directly exposed.