This change updates diff opcodes to use a NamedTuple with definitions and documentation for each element. We had one annoying, confusing thing here, which is that the opcodes returned from the differ had 5 elements, and the opcodes returned from the opcode generator had 6 (including an extra metadata dict). I've changed it so everything operates on the same definition, with the metadata dict being optional in earlier stages of the process. This does mean we have a compatibility break here for the Differ classes, which now yield opcodes as a 6-tuple instead of a 5-tuple. The only place that did this outside of the chunk generator was in the change description diffs for text-based review request fields. In places where we process the opcodes, I've changed it to explicitly use the names instead of unpacking in the loop handler. This means that even where we're continuing to use variables like `i1` and `i2`, we have an initial assignment of those that make it clearer what those variables actually are. There's a lot of other cleanup that's going to happen in the diffviewer code, but I wanted to keep this particular change completely separate. Please don't worry about adjacent code that's unrelated to opcodes. Testing Done: - Did a bunch of smoke testing of diff generation and saw that everything still worked correctly. - Ran unit tests.