Allow calling terminate on workers

[JonasAlaif]

It may not necessarily be possible to break up the computation inside a Worker into nice chunks (e.g. when calling into an external library to do a long-running computation). Thus I haven't been able to find a nice way to force a Worker to stop (dropping the bridge will eventually destroy the worker once computation is done, but will keep running the task until that point). Would it be possible to add support for hard-terminating a Worker using the terminate method?