feat: Add previous_value field to ContractSlot

This change is required by the DCI to properly detect potential address changes (retriggers) on packed storage slots.

substreams/crates/tycho-substreams/src/models.rs

@@ -72,7 +72,7 @@ impl TransactionChangesBuilder {
     }
 
     /// Unique contract/account addresses that have been changed so far.
-    pub fn changed_contracts(&self) -> impl Iterator<Item=&[u8]> {
+    pub fn changed_contracts(&self) -> impl Iterator<Item = &[u8]> {
         self.contract_changes
             .keys()
             .map(|k| k.as_slice())