Wrap function callbacks

[robertgoss]

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Currently function callbacks are passed directly over the interface as arguments.

This leads to 2 issues:

1. We cannot journal calls into these callbacks from within the implementation - which we would like to do in the future
2. This does not follow the hourglass isolation pattern as an exception thown in the callback might not be safely able to percolate in the implementations code.

To get around this we should wrap the function call backs we are passed from the binding in the implementation.

[alexanderoster]

Hi @robertgoss, I just found an issue (#129) that might be directly related to your project. It would be great to hear your opinion about this.