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/builtin.h

@@ -132,6 +132,8 @@ extern const mp_obj_module_t mp_module___main__;
 extern const mp_obj_module_t mp_module_builtins;
 extern const mp_obj_module_t mp_module_sys;
 
+void mp_module_sys_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest);
+
 // Modules needed by the parser when MICROPY_COMP_MODULE_CONST is enabled.
 extern const mp_obj_module_t mp_module_errno;
 extern const mp_obj_module_t mp_module_uctypes;