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/libregex/parse.rs

@@ -37,7 +37,7 @@ pub struct Error {
 
 impl fmt::Show for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f.buf, "Regex syntax error near position {}: {}",
+        write!(f, "Regex syntax error near position {}: {}",
                self.pos, self.msg)
     }
 }