Add a non-canonical SCALE derivations feature

Enables representing IUMT within `StorageValues`. Applied to a variety of
values.

Fixes a bug where `Some([0; 32])` would be considered a valid block anchor.

substrate/primitives/src/merkle.rs

@@ -67,7 +67,8 @@ impl UnbalancedMerkleTree {
 }
 
 /// An unbalanced Merkle tree which is incrementally created.
-#[derive(Clone, PartialEq, Eq, Debug, BorshSerialize, BorshDeserialize)]
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "non_canonical_scale_derivations", derive(scale::Encode, scale::Decode))]
 pub struct IncrementalUnbalancedMerkleTree {
   /// (number of children under branch, branch hash)
   branches: Vec<(u64, [u8; 32])>,