std: Rewrite the `write!` and `writeln!` macros

These are reimplemented using the new `core::fmt` module.

src/libstd/macros.rs

@@ -251,10 +251,17 @@ fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
 /// write!(&mut w, "formatted {}", "arguments");
 /// ```
 #[macro_export]
+#[cfg(not(stage0))]
 macro_rules! write(
     ($dst:expr, $($arg:tt)*) => ({
-        let dst: &mut ::std::io::Writer = $dst;
-        format_args!(|args| { ::std::fmt::write(dst, args) }, $($arg)*)
+        format_args_method!($dst, write_fmt, $($arg)*)
+    })
+)
+#[cfg(stage0)]
+#[macro_export]
+macro_rules! write(
+    ($dst:expr, $($arg:tt)*) => ({
+        format_args!(|args| { $dst.write_fmt(args) }, $($arg)*)
     })
 )
 
@@ -262,9 +269,9 @@ fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
 /// the message is written.
 #[macro_export]
 macro_rules! writeln(
-    ($dst:expr, $($arg:tt)*) => ({
-        let dst: &mut ::std::io::Writer = $dst;
-        format_args!(|args| { ::std::fmt::writeln(dst, args) }, $($arg)*)
+    ($dst:expr, $fmt:expr $($arg:tt)*) => ({
+        format_args!(|args| { $dst.write_fmt(args) },
+                     concat!($fmt, "\n") $($arg)*)
     })
 )