Add support for 'regexp' type and eql comparison of regular expressions.

This pull request adds support for a `regexp` type and equivalence comparisons of regular expressions using `eql`. I'm working on a library that generates regular expressions and I'm using expect.js to do my testing, so having native support for regular expressions would be great. Let me know if you want anything changed in the implementation or if you think it does not belong in core.

I've also updated the tests to account for the new functionality.