This change finishes up modernization of the chunk_generator,

differ, and opcode_generator files in the diffviewer. For the most

part this just involves fixing up docstrings and adding type hints.
The one functional change in here is a fix for a subtle bug that I

discovered when working through this: when computing headers, we were

subtracting 1 from the end line numbers of the chunk twice: once in

_new_chunk, and once in _get_interesting_headers. This meant we'd

stop iterating one line too soon. We obviously didn't notice any bugs in

practice, but it's possible that it was affecting the header

computation, especially for chunks that are only a single line.

Ran unit tests.
Smoke tested a bunch of diffs.