List - native code, based only on Boolean, Pair, Integer

[booch]

We can cheat and implement the constructor in Ruby, because we won't have varargs support for a while.

• constructor
• size, length, empty?
• first, head
• rest, tail
• last
• map, includes?
• reduce, fold, foldl (might need to punt on this)
• foldr (probably need to punt on this, unless we can easily implement reverse)

[booch]

Actually, I think we should make List a built-in function, that calls cons for each element.

[booch]

We should not rely on Pair, but a Church-encoding implementation of cons, first (car), and rest (cdr).