improve lib.rs examples, add tests

[ryan-williams]

This should help with #53 and #54.

[jonaspleyer]

Hey there. I have forked the approx crate under https://crates.io/crates/approxim. I have already included many PRs of the original approx crate such as #57, #67, #69, #83, #84, #87, #90, #91, #92. As you can see, your PR was included as well. This new crate approxim should work as a drop-in-replacement for approx. Feel free to try it out and let me know what you think.