py/objmodule: Add a table of built-in modules with delegation.

This replaces the previous QSTR_null entry in the globals dict which could
leak out to Python (e.g. via iteration of mod.__dict__) and could lead to
crashes.

It results in smaller code size at the expense of turning a lookup into a
loop, but the list it is looping over likely only contains one or two
elements.

To allow a module to register its custom attr function it can use the new
`MP_REGISTER_MODULE_DELEGATION` macro.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/objmodule.h

@@ -28,9 +28,6 @@
 
 #include "py/obj.h"
 
-// Place at the very end of a module's globals_table.
-#define MP_MODULE_ATTR_DELEGATION_ENTRY(ptr) { MP_ROM_QSTR(MP_QSTRnull), MP_ROM_PTR(ptr) }
-
 extern const mp_map_t mp_builtin_module_map;
 extern const mp_map_t mp_builtin_extensible_module_map;