extend the iterator tutorial

documents conversion, size hints and double-ended iterators and adds
more of the traits to the prelude
Remove the global 'vec::to_owned' function
Account for possible 0-sized elements in vector iterators

Closes #7733
auto merge of #7736 : thestinger/rust/doc, r=thestinger

2b96408 r=sanxiyn

documents conversion, size hints and double-ended iterators and adds
more of the traits to the prelude