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/lib.rs

@@ -51,6 +51,27 @@ impl<'a, Resume, Yield, Return> Fiber<'a, Resume, Yield, Return> {
         })
     }
 
+    /// Creates a new fiber with existing stack space that will execute `func`.
+    ///
+    /// This function returns a `Fiber` which, when resumed, will execute `func`
+    /// to completion. When desired the `func` can suspend itself via
+    /// `Fiber::suspend`.
+    ///
+    /// # Safety
+    ///
+    /// The caller must properly allocate the stack space with a guard page and
+    /// make the pages accessible for correct behavior.
+    pub unsafe fn new_with_stack(
+        top_of_stack: *mut u8,
+        func: impl FnOnce(Resume, &Suspend<Resume, Yield, Return>) -> Return + 'a,
+    ) -> io::Result<Fiber<'a, Resume, Yield, Return>> {
+        Ok(Fiber {
+            inner: imp::Fiber::new_with_stack(top_of_stack, func),
+            done: Cell::new(false),
+            _phantom: PhantomData,
+        })
+    }
+
     /// Resumes execution of this fiber.
     ///
     /// This function will transfer execution to the fiber and resume from where