more idiomatic std features

substrate/primitives/Cargo.toml

@@ -15,6 +15,7 @@ untrusted = "0.5"
 twox-hash = "1.1.0"
 byteorder = "1.1"
 blake2-rfc = "0.2.18"
+polkadot-runtime-codec = { path = "../runtime-codec", version = "0.1" }
 
 [dev-dependencies]
 polkadot-serializer = { path = "../serializer", version = "0.1" }
@@ -22,4 +23,4 @@ pretty_assertions = "0.4"
 
 [features]
 default = ["std"]
-std = ["uint/std", "fixed-hash/std"]
+std = ["uint/std", "fixed-hash/std", "polkadot-runtime-codec/std"]