Implement Hash for tuples of up to arity 12. Also change the style to be consistent with core::tuple
auto merge of #16033 : nham/rust/hash_tuple_impl, r=alexcrichton

Previously the implementation of Hash was limited to tuples of up to arity 8. This increases it to tuples of up to arity 12. 

Also, the implementation macro for `Hash` used to expand to something like this:

    impl Hash for (a7,)
    impl Hash for (a6, a7)
    impl Hash for (a5, a6, a7)
    ...

This style is inconsistent with the implementations in core::tuple, which look like this:

    impl Trait for (A,)
    impl Trait for (A, B)
    impl Trait for (A, B, C)
    ...

This is perhaps a minor point, but it does mean the documentation pages are inconsistent. Compare the tuple implementations in the documentation for [Hash](http://static.rust-lang.org/doc/master/std/hash/trait.Hash.html) and [PartialOrd](http://static.rust-lang.org/doc/master/core/cmp/trait.PartialOrd.html)

This changes the Hash implementation to be consistent with `core::tuple`.