Array iteration.

examples/arrays_iter.fun

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+// for-in over an array literal
+for x in [1, 2, 3]
+  print(x)                 // prints: 1, then 2, then 3
+
+// for-in over a named array
+arr = [10, 20, 30]
+for x in arr
+  print(x)                 // prints: 10, then 20, then 30
+
+// nested arrays: iterate rows, then elements
+matrix = [[1, 2], [3, 4]]
+for row in matrix
+  print(row)               // prints: [1, 2] then [3, 4]
+  for v in row
+    print(v)               // prints: 1, 2, 3, 4
+
+// iterate over an array expression (concatenation)
+for v in arr + [40, 50]
+  print(v)                 // prints: 10, 20, 30, 40, 50
+
+// iterate over a slice (end = -1 means till the end)
+for v in arr[1:-1]
+  print(v)                 // prints: 20, 30
+
+// iterate over a function result
+fun make()
+  return [7, 9, 11]
+
+for v in make()
+  print(v)                 // prints: 7, 9, 11