Implement Walker interface

[eric]

One of the neat things about re2 is that you can walk the parsed regular expression to do crazy things with it:

• https://code.google.com/p/re2/source/browse/re2/regexp.h
• https://code.google.com/p/re2/source/browse/re2/walker-inl.h

It would be neat to have access to that in ruby.

[mudge]

Interesting; I've not looked into this part of re2 previously. Do you have a rough idea of how you'd like the API to look or do you just want it to mirror the C++ interface?

[eric]

I don't have a strong feel for what would make the most sense there.

Maybe starting close to the C++ interface would be a good start.