Enabler PHP natives assertion in debug and use Platal::assert() as a

Enabler PHP natives assertion in debug and use Platal::assert() as a
backend when assertions fail.

Thus, you can write the following code:
 assert ('$toto != blah')

If you run in debug mode, this will be evaluated and will raise an
exception in case of error. If you run in production mode, this will juste
be skipped.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>