Add NewString Func

[jackterm]

• Adds NewString() func, which encapsulates the more verbose New().String()
• We use New().String() for testing, and it feels ugly. Rather than adding a wrapper around google/uuid, figured it makes sense to add the utility method to the root package

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[jackterm]

@googlebot I signed it!

[pborman]

Thank you, this looks good and is a nice addition (it is actually the equivalent of gitHub.com/pborman/uuid.New() which was the original package (but now is implemented in terms of this package). I think that was one thing people missed.