Make timer method available on both foreground and background executors

Also, make it return a static future.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/util/src/lib.rs

@@ -123,6 +123,18 @@ where
     }
 }
 
+struct Defer<F: FnOnce()>(Option<F>);
+
+impl<F: FnOnce()> Drop for Defer<F> {
+    fn drop(&mut self) {
+        self.0.take().map(|f| f());
+    }
+}
+
+pub fn defer<F: FnOnce()>(f: F) -> impl Drop {
+    Defer(Some(f))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;