* Strip comments early, before template expansion. This mimics the behaviour in the...

* Strip comments early, before template expansion. This mimics the behaviour in the old parser. Added parser tests demonstrating the regression this fixes. The syntactic effect is fairly elegant, with comments taking effect at source level, as expected. The removeHTMLcomments() and preprocessToDom() passes could be merged at a later date.
* No need for comment stripping in Expr.php anymore
* Updated srvus() to roughly account for these changes
* Gave comment handling its own preprocessor tag, and split off comment handling from extensionSubstitution(). This only applies for the non-HTML modes, since in HTML mode, comments are stripped early.
* Strip comments from template argument names (PPFrame::newChild).