Add manual Default impl for FutureGroup

Removes restriction that `F: Default`

Author: soooch

src/future/future_group.rs

@@ -59,7 +59,6 @@ use crate::utils::{PollState, PollVec, WakerVec};
 /// ```
 
 #[must_use = "`FutureGroup` does nothing if not iterated over"]
-#[derive(Default)]
 #[pin_project::pin_project]
 pub struct FutureGroup<F> {
     #[pin]
@@ -80,6 +79,12 @@ impl<T: Debug> Debug for FutureGroup<T> {
     }
 }
 
+impl<T> Default for FutureGroup<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<F> FutureGroup<F> {
     /// Create a new instance of `FutureGroup`.
     ///