Finally fix std::tuple

[Paolo Carlini]

Chris Jefferson wrote:

> Hopefully this is the final patch to tuple (at least until the results 
> of last week's meetings come out). Yet another invalid symbol (_I this 
> time..)

Hi Chris: I think a name beginning with a capital (after the underscore) 
is preferred, also, a single underscore in names of template parameters. 
Perhaps _Ip? Or whatelse you prefer.

Paolo.