Added all Turtle example variations.
Experiment with optparse-applicative.

Help formatting is too limited.
Add an unregister roshask sub-command.

This command calls ghc-pkg unregister on every package whose name
begins "ROS-" after confirming the user's intent. This is helpful when
upgrading roshask.

Reinstated manual option parsing and help text.
Don't re-generate already registered packages.
Massive code base reorganization.

Moved everything used only by the roshask executable into
src/executable, and stripped off the module path prefix
(i.e. Ros.Core.Msg).

Simplifed the directory structure underneath Ros.

Renamed "Core" to "Internal" everywhere. The word "Core" was
unfortunate as it suggested a relationship with roscore.