pyobject: add conversion methods for single, intX, and uintX types * oct-py-types.cc, oct-py-types.h (pytave::extract_py_uint64): New function. * __py_struct_from_dict__.cc (F__py_uint64_scalar_value__): New function. (F__py_int64_scalar_value__): Simplify and clean up style. Add %!tests. * @pyobject/pyobject.m (pyobject.single, pyobject.int8, pyobject.int16, pyobject.int32, pyobject.uint8, pyobject.uint16, pyobject.uint32, pyobject.uint64): New conversion methods.