Reworked nonstd pointer patch

[Paolo Carlini]

Hi,
> Patch for non-standard pointer support, with all requested changes. 
> Sorry this has taken so long.
>
> All changes requested were made, including renaming _ExtPtr_Allocator.
Not quite ;) I see many unuglifed names, like T, arg, rarg, _U (should
be _Up). Also, the testcases are in nonstd_pointer/, I think now should
be something else, consistent with the rename, like ext_pointer. Also,
we don't constify by value arguments, thus, for example, _FromType *
const arg should be just _FromType* arg. Also make sure that & and *
stay with the type of the argument, e.g., const _ExtPtr_allocator
&__rarg should be const _ExtPtr_allocator& __rarg. Finally, do not
append _ to names, and do not prepend GCC, _EXT_CAST, not _GCC_EXT_CAST.

Last but not least, a ChangeLog entry is missing.

Thanks for your patience,
Paolo.