General:
- Adding integration with PPI::XS, autoloading if installed
Details:
- Added $PPI::XS_COMPATIBLE and $PPI::XS_EXCLUDE variables to guide integration
- Don't autoload PPI::Document, always load
- Load in PPI::XS whenever it is installed
- Loading and depending on Class::Inspector
- PPI::Element::significant implemented in XS (as a trial)