returnStack: use seq[int] instead of Uint256 Stack

nimbus/vm/computation.nim

@@ -144,7 +144,7 @@ proc newComputation*(vmState: BaseVMState, message: Message, salt= 0.u256): Comp
   result.msg = message
   result.memory = Memory()
   result.stack = newStack()
-  result.returnStack = newStack()
+  result.returnStack = @[]
   result.gasMeter.init(message.gas)
   result.touchedAccounts = initHashSet[EthAddress]()
   result.suicides = initHashSet[EthAddress]()

nimbus/vm/interpreter/opcodes_impl.nim

@@ -553,7 +553,7 @@ op returnSub, inline = true:
   # Other than the check that the return stack is not empty, there is no
   # need to validate the pc from 'returns', since we only ever push valid
   # values onto it via jumpsub.
-  c.code.pc = c.returnStack.popInt().safeInt
+  c.code.pc = c.returnStack.pop()
 
 op jumpSub, inline = true, jumpTarget:
   ## 0x5e, Transfers control to a subroutine.
@@ -571,7 +571,7 @@ op jumpSub, inline = true, jumpTarget:
   if c.returnStack.len == 1023:
     raise newException(FullStack, "Out of returnStack")
 
-  c.returnStack.push returnPC
+  c.returnStack.add returnPC
   inc c.code.pc
 
 # ##########################################

nimbus/vm_types.nim

@@ -68,7 +68,7 @@ type
     msg*:                   Message
     memory*:                Memory
     stack*:                 Stack
-    returnStack*:           Stack
+    returnStack*:           seq[int]
     gasMeter*:              GasMeter
     code*:                  CodeStream
     output*:                seq[byte]