Merge from ast-arena.  This reduces the code in Python/ast.c by ~300 lines,
simplifies a lot of error handling code, and fixes many memory leaks.
diff --git a/Include/asdl.h b/Include/asdl.h
index cf05967..a2c86c8 100644
--- a/Include/asdl.h
+++ b/Include/asdl.h
@@ -23,7 +23,7 @@
     void *elements[1];
 } asdl_seq;
 
-asdl_seq *asdl_seq_new(int size);
+asdl_seq *asdl_seq_new(int size, PyArena *arena);
 void asdl_seq_free(asdl_seq *);
 
 #ifdef Py_DEBUG