Tuple helper classes

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#### Tuple helper classes

###### Description

The TR1 Tuple provides 2 helper traits, for compile time access to the tuple size, and the element types.

###### Specification

    tuple_size<T>::value

Requires: `T` is any fusion sequence type, including `tuple`.

Type: MPL Integral Constant

Value: The number of elements in the sequence. Equivalent to ``result_of::size`<T>::type`.

    tuple_element<I, T>::type

Requires: `T` is any fusion sequence type, including `tuple`. `0 <= I < N` or the program is ill formed.

Value: The type of the `I`th element of `T`. Equivalent to ``result_of::value_at`<I,T>::type`.