With this, only main.cpp instantiates an arena. Maybe we'll want to split that up more later (e.g., so that the runtime interpreter uses its own arena), but given the intent to have type-checking update the AST, I thought this was a reasonable approach for now in order to avoid ownership complexities.
Fixes#769
Along with #789 this addresses most of #769 although global_arena is still a TODO (that's widespread and overlaps with other changes so I wanted to do it after these are in).
I think Stack used to use ListNode, but you'd previously removed that dependency. This finalizes the split, and merges ListNode so that we don't accidentally accrue more dependencies. In part I think we'll eventually want to change how Dictionary is set up.
Jon Meow
Jeremy G. Siek