Introduce a new `Visitor` and `Traverser` API

[ryangjchandler]

There used to be a Node trait that all Statement and Expression structures had to implement. This allowed us to have a very small traversal API and cast Node values into a specific Statement or Expression.

The performance of this system wasn't great though and I removed the code that handled it all.

I want to instead introduce a more verbose and performant API for traversing an AST.

The first step is going to be having some sort of Visitor trait that can be implemented on a structure. By default, the trait will have a default set of method implementations for handling all StatementKind and ExpressionKind values.

Visitors will be called by a Traverser where the Traverser is responsible for processing the nodes correctly, i.e. traverse_statement() will match on the kind and call the corresponding visit_X() method.