This adds a deprecation warning to can.Message.extended_id as discussed in #424.

Further, this PR

• specifies when specific deprecated members of can.Message will be removed
• changes len(some_message) to return the DLC instead on len(self.data), so the method also works on remote frames
• makes can.Message(..., check=True) to actually raise a ValueError and not an AssertionError on invalid parameters, as was documented previously; and also make it run when assertions are disabled
• adds small improvements and fixes to can.Message, like Unicode errors being avoided
• adds a hypothesis test for can.Message
• adds minor cleanups to test/*