This change defines a TypedDict for diff chunks, and a tuple for

diff lines, with definitions and documentation for each element.
Before, these were just an opaque dictionary, and a heterogenous list.

The diff lines were particularly annoying because they were variable

length (if there was no metadata, that element was just left off the

end), and types were annoying (for "real" line numbers, we'd have an

int if the line was present on that side of the diff, but an empty

string if not). This change technically breaks compatibility in those

two cases.
We now always include the metadata field, set to None if there's no

metadata for the line. This increases the cached size of a very large

diff with thousands of lines by less that 0.1% (much better than many

years ago with older pickle protocols--the tuple is probably also

helping here).
In the case of real line numbers which are not present on one side of

the diff (for example, the line number of an added line for the

left-hand side of the diff), we set the data for that to None. In

order to maintain compatibility with current versions of Review Bot, we

change None line numbers back to an empty string for the filediff

resource's diff_data mode.
This change only touches code related to chunks and lines. Other diff

viewer cleanup is coming in later changes.

Tested a ton of different diffs, both in the diff viewer as well as

  the text and markdown Review UIs. Verified that all types of edits

  (added/edited/deleted/moved) lines all worked as expected.
Ran unit tests.