bpo-43244: Remove the pyarena.h header (GH-25007)

Remove the pyarena.h header file with functions:

* PyArena_New()
* PyArena_Free()
* PyArena_Malloc()
* PyArena_AddPyObject()

These functions were undocumented, excluded from the limited C API,
and were only used internally by the compiler.

Add pycore_pyarena.h header. Rename functions:

* PyArena_New() => _PyArena_New()
* PyArena_Free() => _PyArena_Free()
* PyArena_Malloc() => _PyArena_Malloc()
* PyArena_AddPyObject() => _PyArena_AddPyObject()

Include/internal/pycore_pyarena.h

@@ -0,0 +1,64 @@
+/* An arena-like memory interface for the compiler.
+ */
+
+#ifndef Py_INTERNAL_PYARENA_H
+#define Py_INTERNAL_PYARENA_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#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 via _PyArena_Malloc().  Pointers to PyObject can
+   also be registered with the arena via _PyArena_AddPyObject(), and the
+   arena will ensure that the PyObjects stay alive at least until
+   _PyArena_Free() is called.  When an arena is freed, all the memory it
+   allocated is freed, the arena releases internal references to registered
+   PyObject*, and none of its pointers are valid.
+   XXX (tim) What does "none of its pointers are valid" mean?  Does it
+   XXX mean that pointers previously obtained via _PyArena_Malloc() are
+   XXX no longer valid?  (That's clearly true, but not sure that's what
+   XXX the text is trying to say.)
+
+   _PyArena_New() returns an arena pointer.  On error, it
+   returns a negative number and sets an exception.
+   XXX (tim):  Not true.  On error, _PyArena_New() actually returns NULL,
+   XXX and looks like it may or may not set an exception (e.g., if the
+   XXX internal PyList_New(0) returns NULL, _PyArena_New() passes that on
+   XXX and an exception is set; OTOH, if the internal
+   XXX block_new(DEFAULT_BLOCK_SIZE) returns NULL, that's passed on but
+   XXX an exception is not set in that case).
+*/
+PyAPI_FUNC(PyArena*) _PyArena_New(void);
+PyAPI_FUNC(void) _PyArena_Free(PyArena *);
+
+/* Mostly like malloc(), return the address of a block of memory spanning
+ * `size` bytes, or return NULL (without setting an exception) if enough
+ * new memory can't be obtained.  Unlike malloc(0), _PyArena_Malloc() with
+ * size=0 does not guarantee to return a unique pointer (the pointer
+ * returned may equal one or more other pointers obtained from
+ * _PyArena_Malloc()).
+ * Note that pointers obtained via _PyArena_Malloc() must never be passed to
+ * the system free() or realloc(), or to any of Python's similar memory-
+ * management functions.  _PyArena_Malloc()-obtained pointers remain valid
+ * until _PyArena_Free(ar) is called, at which point all pointers obtained
+ * from the arena `ar` become invalid simultaneously.
+ */
+PyAPI_FUNC(void*) _PyArena_Malloc(PyArena *, size_t size);
+
+/* This routine isn't a proper arena allocation routine.  It takes
+ * a PyObject* and records it so that it can be DECREFed when the
+ * arena is freed.
+ */
+PyAPI_FUNC(int) _PyArena_AddPyObject(PyArena *, PyObject *);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_PYARENA_H */