add methods to get reads and writes of Access<T> (#3166)

This makes it possible to e.g. show resource and component access on hover in [bevy_mod_debugdump](https://github.com/jakobhellermann/bevy_mod_debugdump/):
![grafik](https://user-images.githubusercontent.com/22177966/142773962-320f6e5b-608e-4abb-88b8-78da4fefc166.png)

crates/bevy_ecs/src/query/access.rs

@@ -115,6 +115,18 @@ impl<T: SparseSetIndex> Access<T> {
             .map(SparseSetIndex::get_sparse_set_index)
             .collect()
     }
+
+    /// Returns all read accesses.
+    pub fn reads(&self) -> impl Iterator<Item = T> + '_ {
+        self.reads_and_writes
+            .difference(&self.writes)
+            .map(T::get_sparse_set_index)
+    }
+
+    /// Returns all write accesses.
+    pub fn writes(&self) -> impl Iterator<Item = T> + '_ {
+        self.writes.ones().map(T::get_sparse_set_index)
+    }
 }
 
 #[derive(Clone, Eq, PartialEq)]