Finish up branch implementation

* while and unconditional loops are now supported fully
* break and continue keywords for loop control
* List::thunkify() has been broken into its own structure so it can be
  broken out further as necessary

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/branch.not

@@ -1,13 +1,9 @@
-check = fn(num) {
-    if num < 10 {
-        println("The number is pretty small.")
-    } elif num < 100 {
-        println("That number is getting pretty big.")
-    } el {
-        println("Whoa! That number is huge!")
-    }
-}
+num = 10
 
-check(1)
-check(10)
-check(100)
+if num < 10 {
+    println("The number is pretty small.")
+} elif num < 100 {
+    println("That number is getting pretty big.")
+} el {
+    println("Whoa! That number is huge!")
+}