objgenerator: Implement return with value and .close() method.

Return with value gets converted to StopIteration(value). Implementation
keeps optimizing against creating of possibly unneeded exception objects,
so there're considerable refactoring to implement these features.

py/objgenerator.h

@@ -0,0 +1 @@
+mp_obj_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_val, mp_obj_t throw_val, mp_vm_return_kind_t *ret_kind);