Implement a concat!() format extension

This extension can be used to concatenate string literals at compile time. C has
this useful ability when placing string literals lexically next to one another,
but this needs to be handled at the syntax extension level to recursively expand
macros.

The major use case for this is something like:

    macro_rules! mylog( ($fmt:expr $($arg:tt)*) => {
        error2!(concat!(file!(), ":", line!(), " - ", $fmt) $($arg)*);
    })

Where the mylog macro will automatically prepend the filename/line number to the
beginning of every log message.

src/test/compile-fail/concat.rs

@@ -0,0 +1,14 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    concat!(foo);   //~ ERROR: expected a literal
+    concat!(foo()); //~ ERROR: expected a literal
+}

src/test/run-pass/concat.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub fn main() {
+    assert_eq!(format!(concat!("foo", "bar", "{}"), "baz"), ~"foobarbaz");
+    assert_eq!(format!(concat!()), ~"");
+
+    assert_eq!(
+        concat!(1, 2i, 3u, 4f32, 4.0, 'a', true, ()),
+        "12344.0atrue"
+    );
+}