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.

Include/pyarena.h

@@ -0,0 +1,42 @@
+/* An arena-like memory interface for the compiler.
+ */
+
+#ifndef Py_PYARENA_H
+#define Py_PYARENA_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+  typedef struct _arena PyArena;
+
+  /* PyArena_New() and PyArena_Free() create a new arena and free it,
+     respectively.  Once an arena has been created, it can be used
+     to allocate memory.  Once it is freed, all the memory it allocated
+     is freed and none of its pointers are valid.
+
+     PyArena_New() returns an arena pointer.  On error, it
+     returns a negative number and sets an exception.
+  */
+  PyAPI_FUNC(PyArena *) PyArena_New(void);
+  PyAPI_FUNC(void) PyArena_Free(PyArena *);
+
+  PyAPI_FUNC(void *) PyArena_Malloc(PyArena *, size_t);
+
+  /* The next two routines aren't proper arena allocation routines.
+     They exist to experiment with the arena API without making wholesale
+     changes to the implementation.
+
+     The two functions register pointers with the arena id.  These
+     are externally allocated pointers that will be freed when the
+     arena is freed.  One takes a pointer allocated with malloc.  The
+     other takes a PyObject that is DECREFed when the arena is freed.
+  */
+  PyAPI_FUNC(int) PyArena_AddMallocPointer(PyArena *, void *);
+  PyAPI_FUNC(int) PyArena_AddPyObject(PyArena *, PyObject *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* !Py_PYARENA_H */