Add support for private virtual functions

[virtuald]

• You can't call them, but you can override them

[virtuald]

Also added a static assert for when a trampoline is abstract, hopefully it's useful?

    build/temp.linux-x86_64-cpython-311/gensrc/rpytest_ft/abstract.cpp:40:71: error: static assertion failed: ::PrivateAbstract has an abstract trampoline -- and they must never be abstract! One of the generated override methods doesn't match the original class or its bases, or is missing. You will need to provide method and/or param overrides for that method. It is likely the following compiler error messages will tell you which one it is.
       40 |     static_assert(std::is_abstract<PrivateAbstract_Trampoline>::value == false, "::PrivateAbstract " RPYBUILD_BAD_TRAMPOLINE);
          |                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
    build/temp.linux-x86_64-cpython-311/gensrc/rpytest_ft/abstract.cpp:40:71: note: the comparison reduces to ‘(1 == 0)’