Replace fake boolean by actual booleans.

I'm actually breaking compat with that, which is not nice, but I hope to fix all those small issues and inconsistencies in the tyxml api for the next release (no deadline, though). 

This removes the nonsensical ```[< | `True | `False]``` and replace it by actual booleans. see joint PR for eliom-widget for the "trigger" of this change.

I will wait a few days before merging this, just to see if someone as some strong arguments against it. :)