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.

crates/wasmtime/Cargo.toml

@@ -72,7 +72,7 @@ experimental_x64 = ["wasmtime-jit/experimental_x64"]
 
 # Enables support for "async stores" as well as defining host functions as
 # `async fn` and calling functions asynchronously.
-async = ["wasmtime-fiber"]
+async = ["wasmtime-fiber", "wasmtime-runtime/async"]
 
 # Enables userfaultfd support in the runtime's pooling allocator when building on Linux
 uffd = ["wasmtime-runtime/uffd"]