Updates with core::fmt changes

1. Wherever the `buf` field of a `Formatter` was used, the `Formatter` is used
   instead.
2. The usage of `write_fmt` is minimized as much as possible, the `write!` macro
   is preferred wherever possible.
3. Usage of `fmt::write` is minimized, favoring the `write!` macro instead.

src/libstd/io/process.rs

@@ -361,8 +361,8 @@ impl fmt::Show for ProcessExit {
     /// Format a ProcessExit enum, to nicely present the information.
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
-            ExitStatus(code) =>  write!(f.buf, "exit code: {}", code),
-            ExitSignal(code) =>  write!(f.buf, "signal: {}", code),
+            ExitStatus(code) =>  write!(f, "exit code: {}", code),
+            ExitSignal(code) =>  write!(f, "signal: {}", code),
         }
     }
 }