Threads created for Fibers/Generators/Enumerator.next must die due to GC and raise a nice error when threads are unavailable

Currently we spin up a thread for fibers, generators, and enumerators that have had next or rewind called on them. This works well functionally, but with a hard reference from the containing object to the created thread, we're creating a situation where even after the fiber/generator/enumerator has been dereferenced, it and anything it references will stay alive because of the spun-up thread.

In addition, there are environments where it will be impossible to spin up threads, and we must produce a friendly error in those cases.

Both issues need to be fixed for this bug, to ensure our use of threads fits as cleanly as possible into all server types.