Chore - Updates to use new API features

[Malivil]

• [x] https://wiki.facepunch.com/gmod/player.Iterator
  • Performance improvement over player.GetAll
  • https://github.com/Facepunch/garrysmod/commit/c14e0fdd65e5f16bd44df9da0043d061dcee4ba6
• [x] https://wiki.facepunch.com/gmod/ents.Iterator
  • Performance improvement over ents.GetAll
• [x] https://wiki.facepunch.com/gmod/net.WritePlayer
  • Performance improvement over net.WriteEntity for players
• [x] https://github.com/Facepunch/garrysmod/pull/1866
  • Usage of net.WriteTable for sequential tables should be updated to use the new sequential parameter AT A MINIMUM
• [x] Change usages of net.Write/ReadString for SteamID64s to the new net.Write/ReadUInt64 to save 80 bits per message

[Malivil]

Completed by:

• 60a4fa64106d177a7b688d0b1b1e01ba816c0831
• 3598e60d876ccbc9a766a2d619adcf0b07c4d422
• ce2d5c44de13b64573ad30458e0e5be259f9f0c0
• 3c2cf1c86bc23e67b8de11178bd717c9df8022d7
• b4a7b8cff17fdbe4324d59687aaecebad39ad361