py/objarray: Allow to create array of void pointers, as extension to CPython.

Using 'P' format specifier (matches struct module). This is another shortcut
for FFI, just as previously introduced "array of objects" ('O').
diff --git a/py/binary.c b/py/binary.c
index d8f865e..71ef3f0 100644
--- a/py/binary.c
+++ b/py/binary.c
@@ -145,6 +145,9 @@
         // Extension to CPython: array of objects
         case 'O':
             return ((mp_obj_t*)p)[index];
+        // Extension to CPython: array of pointers
+        case 'P':
+            return mp_obj_new_int((mp_int_t)((void**)p)[index]);
     }
     return MP_OBJ_NEW_SMALL_INT(val);
 }
@@ -369,5 +372,8 @@
             ((double*)p)[index] = val;
             break;
 #endif
+        // Extension to CPython: array of pointers
+        case 'P':
+            ((void**)p)[index] = (void*)val;
     }
 }