Adds a new mob extension plugin which creates a walk_to() method on
Mobs to allow walking to another entity (accounting for the size of the
entity) with an optional facing position.
Adds a mob-extension plugin, which allows creating modules as mixins to
be monkey-patched into the Mob class. Having a standard method of doing
this prevents confusing problems with mixin name conflicts, by allowing
the plugin to raise an error that 2 mixins with the same name exist.