RFC: Tuple indexing

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As also pointed out [on reddit][reddit], indexing as such (i.e. `tuple[0]`, `tuple[1]` etc.) wouldn't make sense as an alternative, because tuples are heterogenous. (They definitely couldn't be made to implement the `Index`* traits.)

[reddit]: http://www.reddit.com/r/rust/comments/2bll8d/tuples_subscripts/cj6jmm7