This commit changes the definition of Arg so that it knows about which values it can accept, and can display them in the help text. They were already being shown in the help text, but they were passed in separately, so one argument could show two different sets of options if it wanted. Now, the argument itself knows whether there are suggestions, so it doesn’t have to be passed in separately.
This means we can use it for other things, including listing choices when an option is missed out, without having to repeat the list.
With Misfire::BadArgument now only having two fields, it’s not worth using a constructor function anymore.
