Remove anonymous trait parameters, name _ for Rust 2018

https://rust-lang-nursery.github.io/edition-guide/rust-2018/trait-system/no-anon-params.html

protocol/src/types/metadata.rs

@@ -205,7 +205,7 @@ pub enum Value {
 }
 
 pub trait MetaValue {
-    fn unwrap(&Value) -> &Self;
+    fn unwrap(_: &Value) -> &Self;
     fn wrap(self) -> Value;
 }