Diff lines, opcodes, and chunks are now all explicitly typed, and we

made a few tweaks for correctness (such as using None for the line

numbers for the "other" side of an inserted/deleted chunk instead of an

empty string).
This change updates the unit tests that touch these types to make sure

we're always fully defining the structures. Many of these tests used

partial dictionaries for chunks, or even used dictionaries with integer

keys instead of tuples for lines. This of course was flagged by the type

checker.

Ran unit tests.