Implementation of forward_list (and compressed_pair)

[Paolo Carlini]

Chris Fairles wrote:
> Speaking of interfaces, should tuple's get, tuple_element, tuple_size
> etc. be extended for __compressed_pair like they are for std::pair?
>   
I have an answer for this question: I don't think so, I don't think, in
general, we should keep to a minimum dependencies and links between
Standard facilities and extensions.

About the other issue, I'll leave it to Ed. Agreed, if perfecting
__compressed_pair turns out to be too annoying, we can also base
forward_list on tuple, for now, at least...

Paolo.