exec: trigger an exec event

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[alecthomas]

We want exec to be as fast as possiblem as it can get executed many thousands of times a second in some cases. I don't think that is clear if on activate also triggers on exec. I'd be fine with a new hook on exec though, as it is very clear when it applies.

[quad]

I had a feeling you might say something like that. I'll give it a go!

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Closed in favour of #367