Fix help display for non-String RawRepresentables

[natecook1000]

RawRepresentable types that have a non-String raw value are having values displayed in the help screen by converting the RawRep value into a string. However, these values are by default parsed by their raw value, so we should use that for display instead.

This is accomplished by adding a defaultValueDescription implementation for all ExpressibleByArgument-conforming RawValue types, and then basing the allValues implementation on that. This generalizes the existing overloads for String-based RawRep types, while also allowing users who customize their ExpressibleByArgument implementation to provide the correct help value for clients.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test