Implement reverse operator for regular expressions

**Is your feature request related to a problem? Please describe.**
It would be nice for CFG types to allow both left and right recursion. To support this, DFAs must be reversible.
[Reverse of DFA is DFA](https://cs.stackexchange.com/questions/39622/designing-a-dfa-and-the-reverse-of-it)

**Describe the solution you'd like**
Implement a simple `.reverse()` operator for DFAs.
