This lets talents / effects add themselves to lists at appropriate
times. Bascially, just a quicker way than searching for all applied
talents/effects for a specific tag.

Talents can have a sustain_lists and a learn_lists field.
Effects just have a lists field.

Example:
talent A has the following:
learn_lists = {'red', {'blue', 'green'}}
When you learn it, your character now has the following fields:
self.red = {'T_A'}
self.blue.green = {'T_A'}

Modified stealth and step up to use this system. So, to make something
break when step up does, just add it to the 'break_with_step_up' list.