std: Remove io::io_error

* All I/O now returns IoResult<T> = Result<T, IoError>
* All formatting traits now return fmt::Result = IoResult<()>
* The if_ok!() macro was added to libstd

src/libstd/to_bytes.rs

@@ -349,7 +349,7 @@ impl<A:IterBytes> ToBytes for A {
 
         let mut m = ::io::MemWriter::new();
         self.iter_bytes(lsb0, |bytes| {
-            m.write(bytes);
+            m.write(bytes).unwrap();
             true
         });
         m.unwrap()