refactor LibPlayer and LibEntity

[dk1a]

• Encapsulate more of build and entity creation logic in LibEntity
• Rename LibEntity to LibMachineBuild, since it handles machine entity creation, not abstract entity stuff
• Add player entity methods to LibPlayer, particularly getSpawnedPlayerEntity, which can be used in all systems to get the player entity and ensure it's spawned without many lines of overhead
• Use LibMachineBuild.create within LibPlayer.spawn to create machine-like aspects of player
• Change LibUtils.addressToEntityKey(_msgSender()); to LibPlayer.getSpawnedPlayerEntity();, which makes addressToEntityKey mostly obsolete, as it's only used in LibPlayer now

This PR was more of an accidental distraction, hopefully it's still useful

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