Rename ClassManager.OnVisit
Rename ClassManager.OnVisit

To `TraverseTypeClear` to emphasis it's used for type traversal
(similar to tp_traverse)

Addresses comment:
https://github.com/pythonnet/pythonnet/pull/958/#discussion_r390671251

AlexCatarino/pythonnet,AlexCatarino/pythonnet,pythonnet/pythonnet,pythonnet/pythonnet,QuantConnect/pythonnet,AlexCatarino/pythonnet,pythonnet/pythonnet,AlexCatarino/pythonnet,QuantConnect/pythonnet,QuantConnect/pythonnet