UUID Logical Type Support

[noahwill]

I've encountered a case where it would have been helpful to validate that a string in a field, {"type": ["null", {"type": "string", "logicalType": "uuid"}], "default": null}, was in fact a uuid.

To accomplish this, I've added two functions: nativeFromUUID and uuidFromNative. Both of these functions use uuid.Parse to validate that an outgoing or incoming string is a UUID. I went this route rather than using a regex because I found a gist that showed that google's uuid.Parse was much faster. Hopefully, the project is not too opposed to introducing a new package, but I am happy to hear objections.

Please let me know if there's any changes I should make; I hope this is a useful contribution!