libuuid: improve uuid_unparse() performance

libuuid: improve uuid_unparse() performance

There is 2 improvements:

 * remove useless uuid_unpack,
 * directly print the hexa format from memory without using printf
   we can do this as the bytes order is the network byte order
   https://tools.ietf.org/html/rfc4122#section-4.1.2
   even the spatially unique node identifier(the last 6 bytes)

The improvement is important, some results for 1000000 uuid_unparse calls:

Little Endian Ubuntu:
before took 382623 us
after  took  36740 us

Big Endian OpenBSD:
before took 3138172 us
after  took  180116 us

Signed-off-by: Aurelien LAJOIE <orel@melix.net>