Give access to function overloads (ROOT-6094).

Re-implements THashList::GetListForObject() - as if this "THashList" has a gasillion objects and thus each bucket only contains one name.
The lists of overloads are cached to prevent heap allocations unless an update is needed.
PyROOT can now query the overloads if no matching overload is known to it; GetListForObject() will retrieve all currently known overloads of all matching decl contexts and generate TFunction / TMethod objects for them.