This PR adds support for retrieving list information from a slot. Rather
than take the whole list, two different methods are provided for
retrieving a) the length of the list, and b) a specific value in the
list.
- 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.
This turns those functions into general-purpose functions for writing
raw C values into slots on the foreign call stack.
Writing a return just means writing a value to slot 0.
