Demangler interfacd

[Martin von Loewis]

> namespace abi {
> 
>   char* __cxa_demangle(const char* mangled_name,
>                        char* buf,
>                        size_t* n,
>                        int* status);
> 
> }

Is that an extern "C" function? If so, I'd prefer it to be marked as
such (this actually applies to all functions in the draft). 

If not, it does not need the __cxa_ prefix; it then should not have it
if it is user-callable (which it should, since users want to call it
to pretty-print typeid(foo).name()).

Regards,
Martin