Naive effect forwarding

[frank-emrich]

This PR implements naive effect forwarding:

When generating code for resume, we ensure that when asked to handle an effect not considered by the resume instruction under consideration, we simply suspend and let our own parent handle the unknown effect. Once resuming from that we use a back edge to the code that invokes resume on the current fiber (i.e., after moving up in the handler chain to find a handler, we move down in the chain back to where the original suspend of the unknown effect happened).

Payloads are handled as follows:

1. When moving "up", we don't need to consider payloads: They are stored in the "payload buffer" of the VMContext and left untouched while moving upwards in the handler chain until the right handler is found and accesses the data.
2. When moving down, we need to deal with the "tag return payloads" that were provided via resume or cont.bind further up in the chain. These payloads are stored in continuation objects directly. Before resuming into our own child, we thus swap the payload pointers of the current continuation object and the child.

[dhil]

LGTM. Thanks!