Add support for traversing AST using C

[k0va1]

Hi! Prism's C public API doesn't support any traversal features so it's quite tedious to find specific node after parsing(using switch ... case as in the prettyprint_node function). It would be great to have such an opportunity

[kddnewton]

Can you describe a little more about your use case/why you need this in C? All of our binding libraries have visitors.

[k0va1]

Yes, sure. I'm building a tool using plain C so I don't use any bindings for other languages. I understand that it might be out of scope and I've already written a traversal by myself but I think it would be great to have a build-in approach for such thinks because it's quite common task

WDYT?

[kddnewton]

Sure. I've opened a PR

[k0va1]

@kddnewton thanks a lot!