This should make == comparison of UUIDs somewhat faster, and perhaps a
few other operations around maps of UUIDs etc.
FromUUID/ToUUID are used to convert String, which is still used for all
IO of UUIDs. Eventually the hope is those instances can be removed,
and all git-annex branch log files etc use ByteString throughout, for a
real speed improvement.
Note the use of fromRawFilePath / toRawFilePath -- while a UUID usually
contains only alphanumerics and so could be treated as ascii, it's
conceivable that some git-annex repository has been initialized using
a UUID that is not only not a canonical UUID, but contains high unicode
or invalid unicode. Using the filesystem encoding avoids any problems
with such a thing. However, a NUL in a UUID seems extremely unlikely,
so I didn't use encodeBS / decodeBS to avoid their extra overhead in
handling NULs.
The Read/Show instance for UUID luckily serializes the same way for
ByteString as it did for String.
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