Upgrade rust/crates/protobuf to 2.18.1

Test: make
Change-Id: I5b915568dd8c388d36820424b8c7986dfd33af3b
diff --git a/src/reflect/optional.rs b/src/reflect/optional.rs
index ba6cbaf..f81f973 100644
--- a/src/reflect/optional.rs
+++ b/src/reflect/optional.rs
@@ -5,17 +5,17 @@
 use crate::singular::*;
 
 pub trait ReflectOptional: 'static {
-    fn to_option(&self) -> Option<&ProtobufValue>;
+    fn to_option(&self) -> Option<&dyn ProtobufValue>;
 
-    fn set_value(&mut self, value: &ProtobufValue);
+    fn set_value(&mut self, value: &dyn ProtobufValue);
 }
 
 impl<V: ProtobufValue + Clone + 'static> ReflectOptional for Option<V> {
-    fn to_option(&self) -> Option<&ProtobufValue> {
-        self.as_ref().map(|v| v as &ProtobufValue)
+    fn to_option(&self) -> Option<&dyn ProtobufValue> {
+        self.as_ref().map(|v| v as &dyn ProtobufValue)
     }
 
-    fn set_value(&mut self, value: &ProtobufValue) {
+    fn set_value(&mut self, value: &dyn ProtobufValue) {
         match value.as_any().downcast_ref::<V>() {
             Some(v) => mem::replace(self, Some(v.clone())),
             None => panic!(),
@@ -24,11 +24,11 @@
 }
 
 impl<V: ProtobufValue + Clone + 'static> ReflectOptional for SingularField<V> {
-    fn to_option(&self) -> Option<&ProtobufValue> {
-        self.as_ref().map(|v| v as &ProtobufValue)
+    fn to_option(&self) -> Option<&dyn ProtobufValue> {
+        self.as_ref().map(|v| v as &dyn ProtobufValue)
     }
 
-    fn set_value(&mut self, value: &ProtobufValue) {
+    fn set_value(&mut self, value: &dyn ProtobufValue) {
         match value.as_any().downcast_ref::<V>() {
             Some(v) => mem::replace(self, SingularField::some(v.clone())),
             None => panic!(),
@@ -37,11 +37,11 @@
 }
 
 impl<V: ProtobufValue + Clone + 'static> ReflectOptional for SingularPtrField<V> {
-    fn to_option(&self) -> Option<&ProtobufValue> {
-        self.as_ref().map(|v| v as &ProtobufValue)
+    fn to_option(&self) -> Option<&dyn ProtobufValue> {
+        self.as_ref().map(|v| v as &dyn ProtobufValue)
     }
 
-    fn set_value(&mut self, value: &ProtobufValue) {
+    fn set_value(&mut self, value: &dyn ProtobufValue) {
         match value.as_any().downcast_ref::<V>() {
             Some(v) => mem::replace(self, SingularPtrField::some(v.clone())),
             None => panic!(),