Implement default function arguments (for Python functions).

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.
diff --git a/py/compile.c b/py/compile.c
index c1a7955..c4b582e 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -3212,7 +3212,7 @@
         return mp_const_true;
 #else
         // return function that executes the outer module
-        return rt_make_function_from_id(unique_code_id);
+        return rt_make_function_from_id(unique_code_id, MP_OBJ_NULL);
 #endif
     }
 }