Interruptible evals

[darwin]

nREPL has support for interruptible evaluations (e.g interrupting long running Clojure code snippet). I have copy&pasted supporting code from piggieback to dirac nREPL middleware, but never tested it. It won't work for in-flight weasel evaluations anyways, it will leak them I guess. Javascript does not have a good way how to interrupt long-running code.

Because we are using Chrome's debugger protocol for evaluations in dirac, we can interrupt in-flight code executions. This would require an extension of weasel protocol for interruption requests.

I don't want to spend time on it but if anyone would feel that this is important feature to add I'm open for an addition. This task is considered very hard because you would have to understand full stack.