Use proper size_t instead of ssize_t which is not defined in MSVC
Continue iteration even if the first context is empty (part of ROOT-6094).
Roll back workaround of ROOT-6094

proper fix is in.
Remove the stamp and use the ALLHEADERS variable
If the value is not set (due to errors in the runtime evaluation), return default.
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.