Make `SCP_vector`, `SCP_list`, `SCP_set`, and `SCP_multiset` subclasses of their STL counterparts. This allows helper functions to be added to each, including the `contains()` function which is not available until C++20. (This has minor effects on other parts of the codebase that treated `SCP_` and `std::` as synonyms.) Since there is now a nontrivial amount of code that needs to be in a .cpp file, create vmallocator.cpp.
Note that `SCP_string` truncation has been refactored to use length rather than C-string size to be consistent with other `std::basic_string` functions.
Also, remove some enum class hashing code which is no longer necessary now that the codebase is at C++17.
Goober5000
133b033ea0