Bind callback passed to custom events to the node context

[Conduitry]

Fixes #428

[Rich-Harris]

My thinking here was that binding the callback has a performance and memory cost, which could become apparent if you had a very large number of event handlers — you need to create and hold n copies of the function in memory rather than just 1. But it is a little confusing (I've caught myself out that way) and I just realised the documentation gives the wrong advice. So, yeah.

Occurs to me that we could determine at compile time whether or not the callback needs to be bound by seeing whether or not the handler references any data (or this). I'll open a separate issue for that.