Add a tidy check for the license

[brson]

This line should be most everywhere:

  // Copyright 2012 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.

If all files can't be relicensed together then we can have an fxail-license comment to identify them. We'll need to know exactly which files require a license header. For greatest effectiveness, files and directories without the Rust license block should be explicitly whitelisted.

It might be worth it to write this in Rust, as the build system is going that direction.