Add a new stack data type for storing ordered data

[mavnn]

Based on one of the proposals in #967 , this PR implements a new Stack data type for storing an ordered collection of values. I'm putting it up here as a draft for visibility, especially as I'm not even sure if it is a feature that would be considered valuable for the main Inkle project.

Stacks can hold values of any other type, and are not required to be predefined in any way. A stack is initialized using square brackets [ / ] and can optionally contain other values. Note that a global stack cannot be initialized with values stored in other variables.

VAR aStack = ["one", 2]
~aStack += "three" // = [ "one", 2, "three" ]

VAR thing = "thing"
VAR otherStack = [thing] // error

Stacks are "immutable" - this means that you cannot add or remove items from a stack in place, but instead you produce a new stack with the value added or removed. They support the equality operator as well as addition and substraction with slightly unusual properties:

VAR aStack = ["one", "two"]
VAR result = []

// Adding two stacks concatinates them
~result = aStack + ["three", "four"] 
// = ["one", "two", "three", "four"]

// Subtracting from a stack removes the first
// instance of each "equal" value in the second stack
~result = ["one", "one", "two", "two", "one"] - ["one", "two"]
// = ["one", "two", "one"]

// Adding or subtracting an other type from a stack treats the
// other type as if it were a stack holding a single value
~result = aStack + "three"
// = ["one", "two", "three"]

~result = aStack - "two"
// = ["one"] 

To "work through" a stack, we have four native functions. The three pop functions are used to take the newest, oldest, or a random value from the stack while the STACK_COUNT function is used to test whether there any further values to pop.

VAR aStack = ["one", "two"]
VAR result = []
VAR popped = ()

// Pass the stack and a variable reference to a pop function to "pop"
// the next value. The reduced stack is returned as the result of the function.
~result = STACK_POP_NEWEST(aStack, popped)
// result == ["one"], popped == "two"

~result = STACK_POP_OLDEST(aStack, popped)
// result == ["two"], popped == "one"

{STACK_COUNT(aStack)} // == 2

Using these functions, we can also iterate through all the values of a stack if that is useful. This is especially useful in situations where we want to present the player the option to choose one of the values in the stack:

LIST cards = Punch1, Punch2, Kick1, Kick2, Dodge1, Dodge2
VAR hand = []
VAR played = ()

Hello

~hand += Punch1
~hand += Punch1
~hand += Kick1

-> branching(hand) ->

Done! You played {played}!

=== branching(options)
{ STACK_COUNT(options) == 0:
  * ->->
- else:
  ~temp nextOption = ()
  ~temp rest = STACK_POP_NEWEST(options, nextOption)

  <- branching(rest)
  + [{nextOption}]
    ~played += nextOption
    ->->
}

Shows:

Hello

1) Punch1
2) Punch1
3) Kick1

... 1 ...

Done! You played Punch1!