looks like the declaration analysis phase is too early for building type conversion utility code - do it during type analysis

fix cpp_vector_in_generator test by calling create_from_py_utility_code() early enough to let it inject Cython utility code

Add Cython.Compiler.Importer to import code which relies on functionality that is not portable across python versions

removed outdated dummy assignment code that originally prevented an unsed argument gcc warning for __pyx_self, fix one more case where __pyx_self needs to be marked as unused

also use the fast optional kwargs lookup loop when we have **kwargs but receive less than the named optional kwargs