Use reflection to implement TreeDiffer.DiffVisitor.

This is much more robust than overriding individual visitor methods.
It means we won't forget to check some property of an AST node,
and it also means we automatically handle new kinds of nodes.

Using reflection does make this potentially more expensive than having individual visitor methods. The overhead is likely to be small compared to the cost of compiling source code to produce the ASTs in the first place.

Fixes #303.

PiperOrigin-RevId: 494778353