Only need single stack for stacks

[benaadams]

Changes

• Only need a single Stack for stacks as they are always done in pairs so use single one
• Using struct for Stack (that has both arrays) avoids calls to StelemRef to type check from having and Array as the generic param
• Halves the number of operations on Stack and depth book keeping

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No

[Scooletz]

Was looking at this exactly thing 😄

[LukaszRozmej]

lets avoid returning new tuple here, return StackItem directly and add deconstructor to StackItem.