py: Implement core of OrderedDict type.

Given that there's already support for "fixed table" maps, which are
essentially ordered maps, the implementation of OrderedDict just extends
"fixed table" maps by adding an "is ordered" flag and add/remove
operations, and reuses 95% of objdict code, just making methods tolerant
to both dict and OrderedDict.

Some things are missing so far, like CPython-compatible repr and comparison.

OrderedDict is Disabled by default; enabled on unix and stmhal ports.

py/modcollections.c

@@ -31,6 +31,9 @@
 STATIC const mp_map_elem_t mp_module_collections_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR__collections) },
     { MP_OBJ_NEW_QSTR(MP_QSTR_namedtuple), (mp_obj_t)&mp_namedtuple_obj },
+    #if MICROPY_PY_COLLECTIONS_ORDEREDDICT
+    { MP_OBJ_NEW_QSTR(MP_QSTR_OrderedDict), (mp_obj_t)&mp_type_ordereddict },
+    #endif
 };
 
 STATIC MP_DEFINE_CONST_DICT(mp_module_collections_globals, mp_module_collections_globals_table);