Use index instead of pairs in method 'generate' in Fuzz apps

* src/apps/fuzz/ethernet.lua: Iterate with index instead of pairs.
* src/apps/fuzz/ipv4.lua: Iterate with index instead of pairs.
* src/apps/fuzz/ipv6.lua: Iterate with index instead of pairs.
* src/apps/fuzz/matcher.lua: Use ipairs instead of pairs.
* src/apps/fuzz/tcp.lua: Iterate with index instead of pairs.
* src/apps/fuzz/udp.lua: Iterate with index instead of pairs.
* src/core/lib.lua: Add method for copying arrays.