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/mpconfig.h

@@ -839,8 +839,8 @@ typedef double mp_float_t;
 #define MICROPY_STREAMS_POSIX_API (0)
 #endif
 
-// Whether modules can use MP_MODULE_ATTR_DELEGATION_ENTRY() to delegate failed
-// attribute lookups.
+// Whether modules can use MP_REGISTER_MODULE_DELEGATION() to delegate failed
+// attribute lookups to a custom handler function.
 #ifndef MICROPY_MODULE_ATTR_DELEGATION
 #define MICROPY_MODULE_ATTR_DELEGATION (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
 #endif