SSViewer_Scope

This tracks the current scope for an SSViewer instance. It has three goals:

• Handle entering & leaving sub-scopes in loops and withs
• Track Up and Top
• (As a side effect) Inject data that needs to be available globally (used to live in ViewableData)

In order to handle up, rather than tracking it using a tree, which would involve constructing new objects for each step, we use indexes into the itemStack (which already has to exist).

Each item has three indexes associated with it

• Pop. Which item should become the scope once the current scope is popped out of
• Up. Which item is up from this item
• Current. Which item is the first time this object has appeared in the stack

We also keep the index of the current starting point for lookups. A lookup is a sequence of obj calls - when in a loop or with tag the end result becomes the new scope, but for injections, we throw away the lookup and revert back to the original scope once we've got the value we're after