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/lists.wren

@@ -0,0 +1,10 @@
+class Lists {
+  foreign static newList()
+  foreign static insert()
+}
+
+var list = Lists.newList()
+System.print(list is List) // expect: true
+System.print(list.count) // expect: 0
+
+System.print(Lists.insert()) // expect: [4, 5, 6, 1, 2, 3, 9, 8, 7]