rbtools.utils.diffs is used in a lot of diff-generation code, and is

sensitive to string types. These functions are old, lacking modern docs,

and contain positional arguments that are non-obvious.
This change adds type annotations to all the functions in here. It also

updates all documentation for our modern standards, and switches some

parameters to keyword-only (deprecating but still allowing them as

positional arguments).

Unit tests pass on Python 3.7-3.11.