Use iter_mut() instead of mut_iter()

The method `mut_iter()` has been renamed to `iter_mut()` for `MutableSlice`s to comply with naming conventions http://aturon.github.io/style/naming/ownership.html.
Use split_at_mut() instead of mut_split_at()
Merge pull request #1 from b52/master

Use iter_mut() instead of mut_iter()