Use std::quick_exit when exiting

I can't remember the exact details at this point, but during hxcoro development we ran into a problem on cpp when integrating libuv. Roughly, after the program had successfully ran the runtime starts to tear down thread local stuff by running destructors and some gcc specific "on exit" callbacks in libuv. This caused a race with the hxcpp GC being unloaded causing a segfault on program exit.

I think we should start using `std::quick_exit`, both when the user calls `Sys.exit` and when the program completes gracefully. This will prevent the destructors of any thread local stuff being called, which is fine as we're about to exit anyway. If there is something we want to run we can use `std::at_quick_exit`.

Locate the hxcpp runtime entry points for Sys.exit and graceful program completion, then review how they currently tear down threads, the GC, and libuv. Confirm how std::quick_exit and std::at_quick_exit should affect both exit paths, and verify that programs no longer race during shutdown without breaking graceful completion.

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