[move] add a native for reading from the events cacheo

this is useful if we want to understand what event was emitted during
testing, for example.

Author: davidiw

third_party/move/move-vm/runtime/src/data_cache.rs

@@ -295,4 +295,14 @@ impl<'r> TransactionDataCache<'r> {
         let ty_layout = loader.type_to_type_layout(&ty)?;
         Ok(self.event_data.push((guid, seq_num, ty, ty_layout, val)))
     }
+
+    pub(crate) fn emitted_events(&self, guid: Vec<u8>, ty: Type) -> PartialVMResult<Vec<Value>> {
+        let mut events = vec![];
+        for event in self.event_data.iter() {
+            if event.0 == guid && event.2 == ty {
+                events.push(event.4.copy_value()?);
+            }
+        }
+        Ok(events)
+    }
 }

third_party/move/move-vm/runtime/src/native_functions.rs

@@ -156,6 +156,10 @@ impl<'a, 'b, 'c> NativeContext<'a, 'b, 'c> {
         }
     }
 
+    pub fn emitted_events(&self, guid: Vec<u8>, ty: Type) -> PartialVMResult<Vec<Value>> {
+        self.data_store.emitted_events(guid, ty)
+    }
+
     pub fn type_to_type_tag(&self, ty: &Type) -> PartialVMResult<TypeTag> {
         self.resolver.loader().type_to_type_tag(ty)
     }