additional list functions (range, map, filter, reduce, flatten) (#74)

also make the VM cope with reentrancy

docs/system.dj

@@ -624,6 +624,93 @@ The first element is located at index 0.
 
 Indexing out of bounds is an error.
 
+```haku
+range
+  min : number
+  max : number
+  -> list number
+```
+
+Generates a list of integers from `min` to `max` (inclusive).
+
+- When `max > min`, the integers go in an increasing order.
+- When `min > max`, the integers go in a decreasing order.
+
+Note that this function consumes fuel proportional to the amount of elements in the generated list.
+
+```haku
+map
+  l : list t
+  f : \t -> u
+  -> list u
+```
+
+Produces a new list containing all the elements of `l`, run through the function `f` one by one.
+
+```haku
+filter
+  l : list t
+  f : \t -> boolean
+  -> list t
+```
+
+Produces a new list with only those elements of `l` for which `f` returns `True`.
+
+```haku
+reduce
+  l : list t
+  init : a
+  f : \a, t -> a
+  -> a
+```
+
+Reduces the list to a single element, by repeatedly applying a reducing operation `f` on its elements.
+
+This reducing operation receives the current value of an _accumulator_ variable (first argument), and the current element being iterated (second argument).
+Its task is to return the _next_ value of the accumulator variable, which then gets passed into the next element, and so on and so forth.
+
+```haku
+flatten
+  l : list _
+  -> list _
+```
+
+Returns a new list with a single level of nesting flattened.
+
+---
+
+Some of these operations may be a bit confusing, so here are some examples.
+
+```haku
+-- To add two to all elements in a list:
+list = range 1 4  -- [1, 2, 3, 4]
+twoAdded = map list \x ->
+  x + 2
+```
+
+```haku
+-- To filter out only even numbers in a list:
+list = range 1 10
+isEven = \x -> mod x 2 == 0
+onlyEven = filter list isEven
+```
+
+```haku
+-- To sum all the numbers in a list:
+list = [1, 3, 10, 2, 30, 4, 1]
+sum = reduce list 0 \acc, value -> acc + value
+```
+
+```haku
+-- To flatten a singly-nested list:
+list = [[1, 2], [3, 4], [5, 6]]
+flatList = flatten list  -- [1, 2, 3, 4, 5, 6]
+
+-- Note that this only applies to a single level of nesting:
+deepList = [[[1, 2, 3, 4]]]
+lessDeepList = flatten deepList  -- [[1, 2, 3, 4]]
+```
+
 
 ## Shapes