Potential use-after-free and uncollected garbage

[bpcreech]

Note for self (or others!): after #60 there is still an issue wherein in-flight Tasks in the Isolate's foreground runner may either generate garbage for IsolateObjectCollector, or reference torn-down members of the Context object, while the Context is being torn down.

I think the simple fix for this is to stop the IsolateManager's message pump before tearing down the Context, but then we need to remember to clean out the IsolateObjectCollector (which is normally served by the message pump) before we exit.

[bpcreech]

Update: I amended #60 to fix this too.