Replace boost::hash_combine with a lib/hash.h hash_combine performing
the same statement.
Replace inconspicuous global boost hash_value specializations with
std::hash specializations.
No performance difference was observed in three simple MeshManager
measurements.
Remove unused TAG_MASK and h_tag in h_mgr.cpp following 0748c5a75e.
Replace typedef with using keyword and sort header includes.
Differential Revision: https://code.wildfiregames.com/D2441
Tested on: clang 9.0.0, gcc 9.2.0, Jenkins/vs2015, Jenkins/gcc6
This was SVN commit r23191.
