Should we allow event handler to listen to events?

[Mossaka]

If we allow event listening in event handler, this has fundamentally implications to our current design.

First, wasi-cloud uses a pair of two wasmtime::Store, Linker and Instance to run the main program and the handler function. If the function bounces back to main to call another handler function, I assume we should have more wasmtime::Instance instantiated in the host.

Second, this could only be a possibility if multi-handler #68 is implemented.