Add example of new iteration macro

that may replace or complement the loop macro.
The loop macro is still useful though and not
nearly as complicated as the common lisp loop macro.

examples/newiter.janet

@@ -0,0 +1,73 @@
+# Simpler iteration primitives example.
+
+(defn- iter-for
+  [prelude binding start end body]
+  (def $end (gensym))
+  (tuple 'do
+         prelude
+         (tuple 'var binding start)
+         (tuple 'def $end end)
+         (tuple 'while (tuple < binding $end)
+                body
+                (tuple '++ binding))))
+
+(defn- iter-keys
+  [prelude binding tab body]
+  (tuple 'do
+         prelude
+         (tuple 'var binding (tuple next tab nil))
+         (tuple 'while (tuple not= nil binding)
+                body
+                (tuple := binding (tuple next tab binding)))))
+
+(defmacro do-range
+  "Iterate over a half open integer range."
+  [binding start end & body]
+  (def $iter (gensym))
+  (iter-for nil $iter start end
+            (apply tuple 'do (tuple 'def binding $iter) body)))
+
+(defmacro each
+  "Iterate over an indexed data structure."
+  [binding ind & body]
+  (def $iter (gensym))
+  (def $ind (gensym))
+  (iter-for (tuple 'def $ind ind)
+            $iter 0 (tuple length $ind)
+            (apply tuple 'do (tuple 'def binding (tuple get $ind $iter)) body)))
+
+
+(defmacro each-key
+  "Iterate over keys of a table or structure."
+  [binding tab & body]
+  (def $tab (gensym))
+  (def $key (gensym))
+  (iter-keys
+             (tuple 'def $tab tab)
+             $key
+             $tab
+             (apply tuple 'do (tuple 'def binding $key) body)))
+
+(defmacro each-value
+  "Iterate over values of a table or structure."
+  [binding tab & body]
+  (def $tab (gensym))
+  (def $key (gensym))
+  (iter-keys
+             (tuple 'def $tab tab)
+             $key
+             $tab
+             (apply tuple 'do (tuple 'def binding (tuple 'get $tab $key)) body)))
+
+(defmacro each-pair
+  "Iterate over keys and values of a table or structure."
+  [k v tab & body]
+  (def $tab (gensym))
+  (def $key (gensym))
+  (iter-keys
+             (tuple 'def $tab tab)
+             $key
+             $tab
+             (apply tuple 'do 
+                    (tuple 'def k $key)
+                    (tuple 'def v (tuple 'get $tab $key)) body)))