split the Map and Set traits into Map, MutableMap, PersistentMap and Set, MutableSet, PersistentSet

[thestinger]

This needs to be done in order to implement the traits for compile-time maps/sets and persistent maps/sets. There's only std::fun_treemap at the moment and it's not self-balancing yet, so this isn't a high priority.

[emberian]

Triage bump

[bluss]

this was fixed in pull #7768

[sfackler]

#7768 didn't add the Persistent{Map,Set} traits.

[bluss]

I'm sorry, for some reason I didn't see those at all

[thestinger]

I think the Persistent traits can be handled by a different issue report once we actually have useful persistent containers.