Commands' Current Entity within Arc<Mutex<>>

[entropylost]

Commands can be cloned and passed between threads. This makes it so each thread has the same current_entity, which can cause interference; say if one does a .spawn and the other thread does a .with the other thread might pick up the current_entity from the first thread.

I suggest moving the current_entity to be outside of CommandsInternal to solve this problem.

[entropylost]

(And make insert set current_entity while you're at it...)

[cart]

In #798 we removed the arc/mutex from Commands by default, but this issue does still exist for the new Arc<Mutex<Commands>> system parameter. We might need to swap that out for something like ParallelCommands, which would maintain the current entity in each instance.

[MinerSebas]

In 0,5 spawning a new Entity returns an EntityCommands that stores the Entity Id instead.
This means that this Issue can now be closed.

[entropylost]

I have no idea why I wanted this or why it doesn't exist so I'll close it.