Typechecking callbacks

[tolauwae]

Runtime typechecking of the callbacks: do we do this in the callback handling system or in the primitives?

[carllocos]

I propose the primitives because then we just need to type check once: during the call to subscribe_interrupt. Whereas during the callback handling, we would then have to typecheck before handling the events which I believe occurs much more.

Unless it is possible to overwrite in Wasm a table entry to point to another function that has another type signature but I don't think that such thing is possible in Wasm.