Replace be with become

As per rust-lang/rfcs#601, replace `be` with `become` as reserved
keyword for tail call optimization.

src/test/compile-fail/reserved-become.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -9,6 +9,6 @@
 // except according to those terms.
 
 fn main() {
-    let be = 0;
-    //~^ ERROR `be` is a reserved keyword
+    let become = 0;
+    //~^ ERROR `become` is a reserved keyword
 }