bdw-gc coroutines support

Crystal uses bdw-gc and supports coroutines. Fibers is how coroutines are called here. For many years Crystal has been single-thread with fibers. Single-thread is still the default alternative. Sometime ago we added multi-thread support where each thread can run concurrently multiple fibers. This required some patches and eventual contributions to bdw-gc in order to achieve this since there was no built-in support for coroutines in the library.