uuid: Change "fat32" to "fat".

* gnu/system/uuid.scm (%fat32-endianness): Rename to...
(%fat-endianness): ... this.
(fat32-uuid->string): Rename to...
(fat-uuid->string): ... this.
(%fat32-uuid-rx): Rename to..
(%fat-uuid-rx): ... this.
(string->fat32-uuid): Rename to...
(string->fat-uuid): ... this.
(%uuid-parsers, %uuid-printers): Add 'fat16.