py: Add load_const_obj to emitter, add LOAD_CONST_OBJ to bytecode.

This allows to directly load a Python object to the Python stack.  See
issue #722 for background.

py/vm.c

@@ -224,6 +224,12 @@ dispatch_loop:
                     DISPATCH();
                 }
 
+                ENTRY(MP_BC_LOAD_CONST_OBJ): {
+                    DECODE_PTR;
+                    PUSH(ptr);
+                    DISPATCH();
+                }
+
                 ENTRY(MP_BC_LOAD_NULL):
                     PUSH(MP_OBJ_NULL);
                     DISPATCH();