Declare Hashable conformance for ArgumentVisibility

[natecook1000]

In addition to being useful, this allows us to drop the @testable annotation for the ArgumentParser import in ArgumentParserTestHelpers, which resolves #463.

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

[rauhul]

Just so I understand ArgumentVisibility: Hashable is orthogonal to changing the tests to not switch over .base, correct?

[natecook1000]

Right, it's generally useful if the library declaring types like this includes hashability, since clients shouldn't add this kind of conformance.