Move MasKitTests module to masTests.
Rename MasKit enum as Mas.
Upgrade swift-tools-version from 5.3 to 5.6.1.
swift-tools-version 5.5+ is necessary to allow test code to import executable target code,
to allow MasKit library code to be moved into the mas executable.
Upgrade to swift-tools-version to 5.6.1 instead of to 5.5 because they support all the same macOS versions.
Standardize comments.
Signed-off-by: Ross Goldberg <484615+rgoldberg@users.noreply.github.com>
