py: Free non-interned strings in the parser when not needed.

mp_parse_node_free now frees the memory associated with non-interned
strings.  And the parser calls mp_parse_node_free when discarding a
non-used node (such as a doc string).

Also, the compiler now frees the parse tree explicitly just before it
exits (as opposed to relying on the caller to do this).

Addresses issue #708 as best we can.

py/builtinimport.c

@@ -141,7 +141,6 @@ STATIC void do_load(mp_obj_t module_obj, vstr_t *file) {
 
     // compile the imported script
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, false);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         // TODO handle compile error correctly