librustc: Implement simple where clauses.

These `where` clauses are accepted everywhere generics are currently
accepted and desugar during type collection to the type parameter bounds
we have today.

A new keyword, `where`, has been added. Therefore, this is a breaking
change. Change uses of `where` to other identifiers.

[breaking-change]

src/test/auxiliary/where_clauses_xc.rs

@@ -0,0 +1,30 @@
+// Copyright 2014 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 trait Equal {
+    fn equal(&self, other: &Self) -> bool;
+    fn equals<T,U>(&self, this: &T, that: &T, x: &U, y: &U) -> bool
+            where T: Eq, U: Eq;
+}
+
+impl<T> Equal for T where T: Eq {
+    fn equal(&self, other: &T) -> bool {
+        self == other
+    }
+    fn equals<U,X>(&self, this: &U, other: &U, x: &X, y: &X) -> bool
+            where U: Eq, X: Eq {
+        this == other && x == y
+    }
+}
+
+pub fn equal<T>(x: &T, y: &T) -> bool where T: Eq {
+    x == y
+}
+