It makes most sense to name the new 2.7 replacement function to be
_PyOS_double_to_string (with a leading underscore). It will be the same
as PyOS_double_to_string, except that instead of allocating a buffer and
returning it, you will supply it a buffer and a length. That will make
its ultimate replacement easy, if I decide to replace it. And having it
take a buffer and a length means that I don't need to do major surgery
in all of the places that it's called.

In 3.1, there will be no replacement function. All 3.1 code has already
been modified to call PyOS_double_to_string directly.

Since PyOS_ascii_formatd won't be called anywhere in 2.7 or 3.1, I'm
planning on testing its functionality and the fact that it's deprecated
using ctypes.