Added str::char_offset_iter() and str::rev_char_offset_iter()
Renamed bytes_iter to byte_iter to match other iterators
Refactored str Iterators to use DoubleEnded Iterators and typedefs instead of wrapper structs
Reordered the Iterator section
Whitespace fixup
Moved clunky `each_split_within` function to the one place in the tree where it's actually needed
Replaced all block doccomments in str with line doccomments
auto merge of #8082 : Kimundi/rust/master, r=huonw

Also renamed bytes_iter to byte_iter to match other iterators