Add thread safety to at_exit

[ysbaddaden]

Protects the @@handlers class variable of Crystal::AtExitHandlers with a thread mutex.

Avoids a parallel initialization that could create two distinct arrays, and lose one at_exit handler;

Avoids a parallel mutation of the array when at_exit is called in parallel to running the handlers, or when at_exit is called by an at_exit handler 🤷 In these edge cases, the handler is merely added and will be run next.

There should be no behavior change from not using the mutex compared to ST.

I didn't branch based on the preview_mt flag because the Mutex should usually not be contended and its impact be negligible overall. That allowed to keep the implementation simple.

[straight-shoota]

when at_exit is called by an at_exit handler

How would that cause any concurrent mutation?