EIP-3529: Reduce the max gas refunded after a transaction

Previously max gas refunded was defined as gas_used div 2.
Here we name the constant 2 as MAX_REFUND_QUOTIENT and
change its value to 5.

The new equation will be: gas_used div MAX_REFUND_QUOTIENT

nimbus/transaction.nim

@@ -21,10 +21,6 @@ func intrinsicGas*(data: openarray[byte], fork: Fork): GasInt =
       result += gasFees[fork][GasTXDataNonZero]
 
 proc intrinsicGas*(tx: Transaction, fork: Fork): GasInt =
-  const
-    ADDRESS_COST     = 2400
-    STORAGE_KEY_COST = 1900
-
   # Compute the baseline gas cost for this transaction.  This is the amount
   # of gas needed to send this transaction (but that is not actually used
   # for computation)
@@ -34,11 +30,11 @@ proc intrinsicGas*(tx: Transaction, fork: Fork): GasInt =
     result = result + gasFees[fork][GasTXCreate]
 
   if tx.txType > TxLegacy:
-    result = result + tx.accessList.len * ADDRESS_COST
+    result = result + tx.accessList.len * ACCESS_LIST_ADDRESS_COST
     var numKeys = 0
     for n in tx.accessList:
       inc(numKeys, n.storageKeys.len)
-    result = result + numKeys * STORAGE_KEY_COST
+    result = result + numKeys * ACCESS_LIST_STORAGE_KEY_COST
 
 proc getSignature*(tx: Transaction, output: var Signature): bool =
   var bytes: array[65, byte]

nimbus/transaction/call_common.nim

@@ -60,11 +60,6 @@ proc hostToComputationMessage*(msg: EvmcMessage): Message =
     flags:           if msg.isStatic: emvcStatic else: emvcNoFlags
   )
 
-# From EIP-2930 (Berlin).
-const
-  ACCESS_LIST_STORAGE_KEY_COST = 1900.GasInt
-  ACCESS_LIST_ADDRESS_COST     = 2400.GasInt
-
 func intrinsicGas(call: CallParams, fork: Fork): GasInt {.inline.} =
   # Compute the baseline gas cost for this transaction.  This is the amount
   # of gas needed to send this transaction (but that is not actually used
@@ -215,12 +210,18 @@ proc runComputation*(call: CallParams): CallResult =
   else:
     doExec(host, call)
 
+  # EIP-3529: Reduction in refunds
+  let MaxRefundQuotient = if host.vmState.fork >= FkLondon:
+                            5.GasInt
+                          else:
+                            2.GasInt
+
   # Calculated gas used, taking into account refund rules.
   var gasRemaining: GasInt = 0
   if call.noRefund:
     gasRemaining = c.gasMeter.gasRemaining
   elif not c.shouldBurnGas:
-    let maxRefund = (call.gasLimit - c.gasMeter.gasRemaining) div 2
+    let maxRefund = (call.gasLimit - c.gasMeter.gasRemaining) div MaxRefundQuotient
     let refund = min(c.getGasRefund(), maxRefund)
     c.gasMeter.returnGas(refund)
     gasRemaining = c.gasMeter.gasRemaining