Repeated flags fix

[glessard]

Stop failing upon seeing a repeated value from a mutually-exclusive set of flags.

This adds an Equatable requirement to the Flag constructors from CaseIterable enums. Given the other requirements on those enums, Equatable conformance is automatic and therefore costs nothing.

The switch statement in ArgumentSet.updateFlag is also much simplified, by calling values.element(forKey:) only when it is needed.

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

This fixes #59
This supersedes #70

[glessard]

Noting the draft PR #65, specifying the new EnumerableFlag to require Equatable would not be onerous, I think. It would be automatically generated in an overwhelming majority of uses.

[natecook1000]

@glessard This looks great — I think adding the Equatable constraint is fine here, and I'll add that constraint to EnumerableFlag as well.

Can you update with latest and I'll merge?

[glessard]

I rebased and squashed. Thanks!