Fill NewV7 from byte 6 instead of 7

I was off by 1 in my head. Bytes 0-5 (len 6) are the timestamp
and then bytes 6-15 (len 10) are random data.

Add check to Parse for valid `urn:uuid` prefix

Check that a `urn:uuid` actually has the corrent prefix. Prevously any
45 length UUID would be parsed (which could be invalid).

Also validate that the version bits are one of the supported versions (4 or 7)
and check the variant is RFC4122.

Resolves #3

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