Embedder API for continuations

[dhil]

Implementing an interface for continuations in the wasmtime crate would enable us to use continuation-y things in an embedded context.

[dhil]

In #161 I noticed that part of the coredump facility makes use of the embedder API. Currently, we hack around the lack of embedder support for continuation by mapping them to funcrefs. Though, it would be good to have bespoke support for continuations to avoid unpleasant surprises.