auto merge of #5447 : erickt/rust/incoming, r=graydon

This normalizes the how we call reverse iteration functions. It also adds a char_len method, and a method to iterate backwards over a string.

Author: bors

src/libcore/repr.rs

@@ -218,7 +218,7 @@ pub impl ReprVisitor {
 
     fn write_escaped_slice(&self, slice: &str) {
         self.writer.write_char('"');
-        for str::chars_each(slice) |ch| {
+        for slice.each_char |ch| {
             self.writer.write_escaped_char(ch);
         }
         self.writer.write_char('"');