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/src/trampoline/func.rs

@@ -276,7 +276,7 @@ pub fn create_function(
 
     unsafe {
         Ok((
-            OnDemandInstanceAllocator::new(None).allocate(InstanceAllocationRequest {
+            OnDemandInstanceAllocator::default().allocate(InstanceAllocationRequest {
                 module: Arc::new(module),
                 finished_functions: &finished_functions,
                 imports: Imports::default(),
@@ -308,7 +308,7 @@ pub unsafe fn create_raw_function(
     finished_functions.push(func);
 
     Ok(
-        OnDemandInstanceAllocator::new(None).allocate(InstanceAllocationRequest {
+        OnDemandInstanceAllocator::default().allocate(InstanceAllocationRequest {
             module: Arc::new(module),
             finished_functions: &finished_functions,
             imports: Imports::default(),