keep track of the original generator function for debugging

I'm developing a Firefox devtool addon that is a channel debugger. It graphs a timeline of events by instrumenting the code with breakpoints using our debugger API. It's coming along really well.

The one thing that I haven't been able to do is accurately name each process. I can do it within `go` (set a breakpoint there, inspect the function passed in, get the name of it). But that doesn't work if a process is currently running and then you press "record"; it will get events from a process but because we only have a generator instance I have no idea what name of the original function is.

I propose this simple change which will greatly help with the debugger. I can't think of any implications of this; possibly the function won't be garbage collected until the process dies but I bet that happens anyway with generators.

Here's a screenshot for fun (lots more to do):

![screen shot 2014-10-29 at 6 36 27 pm](https://cloud.githubusercontent.com/assets/17031/4835944/05e26668-5fbd-11e4-8cf2-fc5003dc96c9.png)