[Patch] Use variadic templates for tr1::is_function

[Paolo Carlini]

Hi all, hi Doug,

I'm finishing regtesting the below, which I consider better than 
instantiating a lot of templates for a single trait and/or using 
something like the old __in_array. Do you agree?

Besides that, I'm a little puzzled by this kind of code in tr1/functional:

  /**
   * @if maint
   * Retrieve the result type for a function type.
   * @endif
   */
  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes...)>
    {
      typedef _Res result_type;
    };

and wanted to quickly double check with you that variadic functions are 
left out on purpose...

Thanks,
Paolo.

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