RUST-993 Implement Clone for Collection<T> (mongodb#454)

Because derive is too conservative, derive only implements Clone for Collection<T> if T is Clone.
Collection<T> does not actually store any value of type T (so T does not need to be Clone).

src/coll/mod.rs

@@ -111,12 +111,23 @@ use crate::{
 /// # }
 /// ```
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct Collection<T> {
     inner: Arc<CollectionInner>,
     _phantom: std::marker::PhantomData<T>,
 }
 
+// Because derive is too conservative, derive only implements Clone if T is Clone.
+// Collection<T> does not actually store any value of type T (so T does not need to be clone).
+impl<T> Clone for Collection<T> {
+    fn clone(&self) -> Self {
+        Self {
+            inner: self.inner.clone(),
+            _phantom: Default::default(),
+        }
+    }
+}
+
 #[derive(Debug)]
 struct CollectionInner {
     client: Client,