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

src/libstd/str/ascii.rs

@@ -115,7 +115,7 @@ impl<'self> AsciiCast<&'self[Ascii]> for &'self str {
 
     #[inline]
     fn is_ascii(&self) -> bool {
-        self.bytes_iter().all(|b| b.is_ascii())
+        self.byte_iter().all(|b| b.is_ascii())
     }
 }