refactor(contracts): combine two mappings into one with a struct

packages/contracts/contracts/L1/rollup/CanonicalTransactionChain.sol

@@ -229,7 +229,7 @@ contract CanonicalTransactionChain is ICanonicalTransactionChain, Lib_AddressRes
             uint40
         )
     {
-        return uint40(transactionHashes.length);
+        return uint40(queueElements.length);
     }
 
     /**

packages/contracts/test/contracts/L1/rollup/CanonicalTransactionChain.gas.spec.ts

@@ -218,7 +218,7 @@ describe('[GAS BENCHMARK] CanonicalTransactionChain', () => {
         'Non-calldata overhead gas cost per transaction:',
         (gasUsed - fixedCalldataCost) / numTxs
       )
-      expectApprox(gasUsed, 1_617_381, {
+      expectApprox(gasUsed, 1_619_781, {
         absoluteUpperDeviation: 1000,
         // Assert a lower bound of 1% reduction on gas cost. If your tests are breaking because your
         // contracts are too efficient, consider updating the target value!
@@ -309,7 +309,7 @@ describe('[GAS BENCHMARK] CanonicalTransactionChain', () => {
       console.log('Benchmark complete.')
       printGasSavings(gasUsed, 237_065)
 
-      expectApprox(gasUsed, 206_880, {
+      expectApprox(gasUsed, 187_081, {
         absoluteUpperDeviation: 500,
         // Assert a lower bound of 1% reduction on gas cost. If your tests are breaking because your
         // contracts are too efficient, consider updating the target value!
@@ -331,7 +331,7 @@ describe('[GAS BENCHMARK] CanonicalTransactionChain', () => {
       console.log('Benchmark complete.')
       printGasSavings(gasUsed, 196_457)
 
-      expectApprox(gasUsed, 146_499, {
+      expectApprox(gasUsed, 126_700, {
         absoluteUpperDeviation: 500,
         // Assert a lower bound of 1% reduction on gas cost. If your tests are breaking because your
         // contracts are too efficient, consider updating the target value!