Implement allocating fiber stacks for an instance allocator.

This commit implements allocating fiber stacks in an instance allocator.

The on-demand instance allocator doesn't support custom stacks, so the
implementation will use the allocation from `wasmtime-fiber` for the fiber
stacks.

In the future, the pooling instance allocator will return custom stacks to use
on Linux and macOS.

On Windows, the native fiber implementation will always be used.

crates/fiber/src/windows.rs

@@ -40,7 +40,7 @@ where
 }
 
 impl Fiber {
-    pub fn new<F, A, B, C>(stack_size: usize, func: F) -> io::Result<Fiber>
+    pub fn new<F, A, B, C>(stack_size: usize, func: F) -> io::Result<Self>
     where
         F: FnOnce(A, &super::Suspend<A, B, C>) -> C,
     {
@@ -61,11 +61,19 @@ impl Fiber {
                 drop(Box::from_raw(state.initial_closure.get().cast::<F>()));
                 Err(io::Error::last_os_error())
             } else {
-                Ok(Fiber { fiber, state })
+                Ok(Self { fiber, state })
             }
         }
     }
 
+    pub fn new_with_stack<F, A, B, C>(_top_of_stack: *mut u8, _func: F) -> Self
+    where
+        F: FnOnce(A, &super::Suspend<A, B, C>) -> C,
+    {
+        // Windows fibers have no support for custom stacks
+        unimplemented!()
+    }
+
     pub(crate) fn resume<A, B, C>(&self, result: &Cell<RunResult<A, B, C>>) {
         unsafe {
             let is_fiber = IsThreadAFiber() != 0;