Hide OpaqueExtrinsic internal and add builder

primitives/runtime/src/generic/unchecked_extrinsic.rs

@@ -326,7 +326,7 @@ where
 	Extra: SignedExtension,
 {
 	fn from(extrinsic: UncheckedExtrinsic<Address, Call, Signature, Extra>) -> Self {
-		OpaqueExtrinsic::decode(&mut extrinsic.encode().as_slice())
+		OpaqueExtrinsic::from_bytes(extrinsic.encode().as_slice())
 			.expect(
 				"both OpaqueExtrinsic and UncheckedExtrinsic have encoding that is compatible with \
 				raw Vec<u8> encoding; qed"

primitives/runtime/src/lib.rs

@@ -714,7 +714,14 @@ macro_rules! assert_eq_error_rate {
 /// Simple blob to hold an extrinsic without committing to its format and ensure it is serialized
 /// correctly.
 #[derive(PartialEq, Eq, Clone, Default, Encode, Decode)]
-pub struct OpaqueExtrinsic(pub Vec<u8>);
+pub struct OpaqueExtrinsic(Vec<u8>);
+
+impl OpaqueExtrinsic {
+	/// Convert an encoded extrinsic to an `OpaqueExtrinsic`.
+	pub fn from_bytes(mut bytes: &[u8]) -> Result<Self, codec::Error> {
+		OpaqueExtrinsic::decode(&mut bytes)
+	}
+}
 
 #[cfg(feature = "std")]
 impl parity_util_mem::MallocSizeOf for OpaqueExtrinsic {