Replace Generex with RgxGen

[toddobryan]

Generex https://github.com/mifmif/Generex seems to be moribund. The last commit was over 4 years ago. It has problems--for example, it treats ^ and $ as literals, so it thinks "^abc$" is a string that matches the pattern ^[a-z]{3}$. (It's not.)

RgxGen https://github.com/curious-odd-man/RgxGen seems to be still under development, handles ^ and $ and character classes like \w, \S, and other things, and claims to be faster than Generex and Xeger.

RgxGen can also generate non-examples, if that's ever useful.

[sksamuel]

I like. We should do this.

[sksamuel]

Now merged in master.