Added iterators

Iterators are an unbounded sequence of elements that can only be consumed one-by-one.

_docs/mod/itrs.md

@@ -0,0 +1,20 @@
+---
+---
+
+# Iterator Builtins
+
+### from
+
+```
+itrs:from LIST
+```
+
+Returns an iterator which will step through the elements of LIST.
+
+### to-list
+
+```
+lists:to-list ITR
+```
+
+Consume the elements of the iterator ITR and return the elements as a list.