add StringSet.HasAny

[deads2k]

HasAny is useful shorthand. We use it for determining whether any change is necessary: https://github.com/deads2k/origin/blob/deads-make-remove-user-predictable/pkg/cmd/experimental/policy/remove_user_from_project.go#L74

@derekwaynecarr Any objection to creating this in the base library?

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[derekwaynecarr]

No objection from me... I know I added Difference, so HasAny is useful. Add a test case.

[deads2k]

unit test added.

[smarterclayton]

LGTM