Split out fiber stacks from fibers.

This commit splits out a `FiberStack` from `Fiber`, allowing the instance
allocator trait to return `FiberStack` rather than raw stack pointers. This
keeps the stack creation mostly in `wasmtime_fiber`, but now the on-demand
instance allocator can make use of it.

The instance allocators no longer have to return a "not supported" error to
indicate that the store should allocate its own fiber stack.

This includes a bunch of cleanup in the instance allocator to scope stacks to
the new "async" feature in the runtime.

Closes #2708.

Cargo.lock

@@ -3484,6 +3484,7 @@ dependencies = [
  "thiserror",
  "userfaultfd",
  "wasmtime-environ",
+ "wasmtime-fiber",
  "winapi",
 ]