Implement SetStrict with tests

This PR partially resolves #11 with an implementation of Set based on an array. It's called "SetStrict" here but could be renamed to something better. It has accompanying tests and is 100% covered.

SetStrict should be used as an alternative to Set when object uniqueness is necessary, or if using functions in a  set.

To be clear, #11 should still be open until the implementation with JSON.stringify suggested by @peterkhayes is implemented.