Update parity-scale-codec to 2.0 (#7994)

* update cargo.toml

* use 2.0 in mmmr

substrate/primitives/arithmetic/src/per_things.rs

@@ -338,9 +338,9 @@ macro_rules! implement_per_thing {
 			fn encode_as(&self) -> &Self::As {
 				&self.0
 			}
-			fn decode_from(x: Self::As) -> Self {
+			fn decode_from(x: Self::As) -> Result<Self, codec::Error> {
 				// Saturates if `x` is more than `$max` internally.
-				Self::from_parts(x)
+				Ok(Self::from_parts(x))
 			}
 		}