fail!() used to require owned strings but can handle static strings
now. Also, it can pass its arguments to fmt!() on its own, no need for
the caller to call fmt!() itself.
This will allow you to use the + operator to add together any two
Options, assuming that the contents of each Option likewise implement
+. So Some(4) + Some(1) == Some(5), and adding with None leaves the
other value unchanged.
This might be monoidic? I don't know what that word means!
