Provide better error messages for interface violations.

Close #5.
Merge branch 'master' of github.com:BurntSushi/toml-test
On second thought, I'm going to remove this.

Having two different ways to run tests just doesn't seem like a good
idea to me. They can produce different output. toml-test was built to do
a thorough comparison that cannot be done with string equality and
thereby produce better witnesses.