sp-runtime: make parity-util-mem dependency optional (#12657)

* `sp-runtime`: make `parity-util-mem` dependency optional

* Use default-features = false for sp-runtime in sp-keyring

* Remove parity-util-mem from sp-core

* Cargo.lock

* Restore default-features for keyring dependency

substrate/primitives/runtime/src/transaction_validity.rs

@@ -226,9 +226,8 @@ impl From<UnknownTransaction> for TransactionValidity {
 /// Depending on the source we might apply different validation schemes.
 /// For instance we can disallow specific kinds of transactions if they were not produced
 /// by our local node (for instance off-chain workers).
-#[derive(
-	Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug, parity_util_mem::MallocSizeOf,
-)]
+#[derive(Copy, Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug)]
+#[cfg_attr(feature = "std", derive(parity_util_mem::MallocSizeOf))]
 pub enum TransactionSource {
 	/// Transaction is already included in block.
 	///