Add the unit tests

[briansmith]

In my private branch of ring there are tests for various users of untrusted.rs, but there's no unit tests in this repo. This is the exactly the kind of project that should have 100% code coverage of the highest sort.

[briansmith]

Corey Converted some examples he made into tests and they've landed. I think they'll be a good example to follow for others that want to add more cases to test the various edge cases of the functions. Thanks @frewsxcv!

[duesee]

Maybe this information is helpful for someone. There is an AFL adaption to Rust programs available, see https://github.com/frewsxcv/afl.rs.

[berendsliedrecht]

Is this still wanted? If so, I would not mind picking it up. Are there any specifics that need testing, or available test vectors?