file-systems: Introduce (gnu system uuid).
* gnu/build/file-systems.scm (sub-bytevector)
(latin1->string, %fat32-endianness, fat32-uuid->string)
(%iso9660-uuid-rx, string->iso9660-uuid)
(iso9660-uuid->string, %network-byte-order)
(dce-uuid->string, %uuid-rx, string->dce-uuid)
(string->ext2-uuid, string->ext3-uuid, string->ext4-uuid)
(vhashq, %uuid-parsers, %uuid-printers, string->uuid)
(uuid->string): Move to...
* gnu/system/uuid.scm: ... here. New file.
* gnu/system/file-systems.scm (uuid): Move to the above file.
* gnu/system/vm.scm: Adjust accordingly.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add uuid.scm.
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