Memory leaks in RootSystem

[tscrim]

RootSystem is currently creating memory leaks in the following way:

Upon creating a root system, say of type B3, this line:

self.dual = RootSystem(self._cartan_type.dual(), as_dual_of=self);

causes the B3 root system to be used as a key in the UniqueRepresentation of the C3 root system (I call it so, but note it is the dual of the B3, which will be different than the honest C3 root system), which is a strong reference that can never be deleted. Moreover, the B3 root system then holds a (strong) reference to the C3 root system, so the C3 root system never gets collected either.

Some data:

sage: from collections import Counter
sage: import gc
sage: gc.collect()
91
sage: pre = {id(a) for a in gc.get_objects()}
sage: get_memory_usage()
1022.7890625
sage: for n in range(5,3000):
    RS = RootSystem(['A',n])
....:     
sage: gc.collect()
0
sage: get_memory_usage()
1031.359375
sage: post = Counter(str(type(a)) for a in gc.get_objects() if id(a) not in pre)
sage: [p for p in post.iteritems() if p[1] > 2000]
[("<class 'weakref.KeyedRef'>", 8985),
 ("<class 'sage.combinat.root_system.type_A.CartanType'>", 2994),
 ("<class 'sage.combinat.root_system.root_system.RootSystem'>", 5990),
 ("<type 'dict'>", 6012),
 ("<type 'tuple'>", 29952)]