Can't use holders around types with custom converters

This fails for any T with a custom type converter (e.g. std::string, Eigen::MatrixXd, MyCustomConvertingType, etc.):

m.def("f", [](std::shared_ptr<T>) {})

The issue is that the holder type casters assume that type_caster<T> is a type_caster_base<T>, and make use of type_caster_base<T> constructors and internals. #786 makes it into a compile-time failure (before that it will result in strange run-time failures as we end up invoking a generic caster on an unregistered type).

Start with the holder type casters and the type_caster assumptions described in the issue, then reproduce the compile-time failure using the shared_ptr binding snippet with a custom converter. Done means holders work with custom-converted types without relying on type_caster_base internals or invoking a generic caster for an unregistered type.