std: more dummy type parameters on iterators to work around #6967.
std: remove vec::each2 and vec::each2_mut in favour of iterators
std: remove each[i]_mut functions, in favour of iterators.
auto merge of #6995 : huonw/rust/each-fn-kill, r=thestinger

This kills the `each2`, `each2_mut`, `each_mut` and `eachi_mut` functions in `std::vec` and replaces them with the appropriate `std::iterator` incantation.

(The `each_mut` etc methods from `old_iter` are left untouched, so far.)