hash: switch to CC0 license.

As per email from Andreas (the only other contributor):
From: Andreas Schlick <schlick@lavabit.com>
Subject: Re: Changing from Public Domain to CC0
To: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 01 Nov 2012 12:51:47 +0100

Hi!

> Since Public Domain is a slippery concept outside certain countries,
> it has been suggested that I formalize it to Creative Commons Zero.
>
> I want to change the License string (and add a symlink) to the
> following trivial modules.  Please Ack.

Sure, ack.

 Andreas

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>