[cpyrt] Take element type of std::initializer_list args from the class

[guitargeek]

Almost every constructor of a standard container has a std::initializer_list argument, and each of them resolved a type from its spelling, which declares new code:

• GetPriority() resolved the argument name through GetScope(), which deliberately maps std::initializer_list to std::vector, a mapping the ranking of overloads relies on. This declared a new trampoline variable on every call.
• CreateConverter() and the InitializerListConverter resolved the element type T from a substring of the type name.

Take T from the template arguments of the std::initializer_list class instead, and instantiate std::vector from it directly for the ranking. Names like std::vector<std::initializer_list>, where the list is not the argument class itself, still go through GetScope().

Add interop wrappers for Cpp::GetClassTemplateInstantiationArgs() and Cpp::GetUnderlyingType().

For example:

    import time
    import cppjit

    cppjit.cppdef("#include <set>\n#include <list>\n#include <vector>\n#include <utility>\n")

    types = ["int", "long", "short", "char", "float", "double",
             "unsigned int", "unsigned long"]
    pairs = [f"std::pair<{a}, {b}>" for a, b in
             [("int", "int"), ("long", "double"), ("double", "double"),
              ("short", "short"), ("float", "float"), ("long", "long"),
              ("unsigned int", "unsigned long"), ("char", "char")]]

    classes = ([cppjit.gbl.std.set[t] for t in types] +
               [cppjit.gbl.std.list[t] for t in types] +
               [cppjit.gbl.std.vector[p] for p in pairs])

    start = time.perf_counter()
    for cls in classes:
        cls()  # first construction of each class
    print(f"first construction of {len(classes)} container classes: {1000 * (time.perf_counter() - start):.0f} ms")

This takes about 645 ms instead of 690 ms (median of 3 trials, Linux x86_64, Python 3.14, CppInterOp/LLVM 22.1.8). The savings grow with the number of distinct container instantiations created for the first time.