Add C API functions for working with lists:

- wrenEnsureSlots()
  Lets you go the foreign slot stack to make room for a temporary work
  area.

- wrenSetSlotNewList()
  Creates a new empty list and stores it in a slot.

- wrenInsertInList()
  Takes a value from one slot and inserts it into the list in another.

Still need more functions like getting elements from a list, removing,
etc. but this at least lets you create, populate, and return lists from
foreign methods.

test/api/slots.wren

@@ -1,9 +1,8 @@
 class Slots {
   foreign static noSet
-
   foreign static getSlots(bool, num, string, bytes, value)
-
   foreign static setSlots(a, b, c, d)
+  foreign static ensure()
 }
 
 // If nothing is set in the return slot, it retains its previous value, the
@@ -14,3 +13,5 @@ var value = ["value"]
 System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value) // expect: true
 
 System.print(Slots.setSlots(value, 0, 0, 0) == value) // expect: true
+
+System.print(Slots.ensure()) // expect: 1 -> 20 (190)