Implement the ability to start Entity Threads from C++ script threads

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This would be for both ES and C++ threads. On the C++ side, we could support arbitrary function objects and coroutines/fibers.

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This is now implemented for:

• Embedded scripts via SCRIPT_BEGIN + SCRIPT_END
• Lambdas acting as coroutines (possibly unsafe -- see Fix API safety for control-flow awaitables like until when used from tangentially related fibers #78)
• Arbitrary fibers to be encapsulated by an EntityThread instance
• Script object instances passed by rvalue reference (ownership given to bootstrap function)

We currently need safety issues like #78 to be resolved, as well as #79 to be completed in order to close this.

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start_thread and start_script now work with thread ID/name hashes, Script type as template parameter, and Script instance as entry-point. Last item would be #78 to close this out.