auto merge of #8446 : alexcrichton/rust/ifmt++, r=graydon

This includes a number of improvements to `ifmt!`

* Implements formatting arguments -- `{:0.5x}` works now
* Formatting now works on all integer widths, not just `int` and `uint`
* Added a large doc block to `std::fmt` which should help explain what `ifmt!` is all about
* Added floating point formatters, although they have the same pitfalls from before (they're just proof-of-concept now)

Closed a couple of issues along the way, yay! Once this gets into a snapshot, I'll start looking into removing all of `fmt`

src/libstd/unstable/raw.rs

@@ -56,6 +56,7 @@ impl<'self, T> Repr<Slice<T>> for &'self [T] {}
 impl<'self> Repr<Slice<u8>> for &'self str {}
 impl<T> Repr<*Box<T>> for @T {}
 impl<T> Repr<*Box<Vec<T>>> for @[T] {}
+impl Repr<*String> for ~str {}
 
 // sure would be nice to have this
 // impl<T> Repr<*Vec<T>> for ~[T] {}