This version of the commit has the on_ceiling/on_wall fix without any
 explaination of that code, since apparently it wasn't complicated enough.

As for the notes at the top of the function, they're still there.

move_and_slide is rather useful for character-controllers, etc.
It reduces the amount of boilerplate code.
Not having move_and_slide makes the APIs somewhat inconsistent.

(It might be nice to figure out a way to share the code between the two
move_and_slide implementations, but that's for someone who knows what
the policy is on shared code like that.)