Trim the fiber interface/implementation

[dhil]

Currently, we build the stack switching logic on top of Wasmtime Fiber. The interface (and implementation) provided by Wasmtime Fiber is more general and conservative than we require. We should trim the interface and implementation down to a bare minimum. As a result, we ought to see a some performance improvements too, because, in our case, Wasmtime Fiber performs some redundant writes and causes two unnecessary indirections on each stack switch.

[dhil]

@frank-emrich one of your previous PRs trimmed down the interface of fibre considerably, right?