Added iterators

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

_docs/mod/core.md

@@ -74,14 +74,17 @@ Returns the length of COL. If COL is a list or hash, COL will be the number of
 elements. If COL is a string, COL will be the string's length. All other values will
 return a length of 0.
 
+If COL is an iterator, `len` will consume the values of the iterator and return the number of items consumed. 
+
 ### map
 
 ```
 map COL BLOCK
 ```
 
-Returns a new list of elements mapped from COL according to the result of BLOCK. COL can be any listable data
-structure, however the result will always be a concrete list.
+Returns a new list of elements mapped from COL according to the result of BLOCK. COL can be any list or hash
+with the result being a concrete list. COL can be an iterator, in which case the result will be an iterator
+which will call BLOCK for every consumed value.
 
 ```
 map [1 2 3] { |x| str $x | len }
@@ -107,7 +110,7 @@ reduce COL [INIT] BLOCK
 Returns the result of reducing the elements of COL with the passed in block.
 
 BLOCK will receive at least two argument, with the current value of the accumulator always being the last argument.
-If COL is a list, the arguments will be _|element accumulator|_, and if COL is a hash, the arguments will be
+If COL is a list or iterator, the arguments will be _|element accumulator|_, and if COL is a hash, the arguments will be
 _|key value accumulator|_. 
 
 The block result will be set as the value of the accumulator for the next iteration. Once all elements are process

_docs/mod/index.md

@@ -5,4 +5,5 @@ Modules of the standard library:
 - [core](/mod/core): Core builtins
 - [csv](/mod/csv): Functions for operating over CSV data.
 - [fs](/mod/fs): File system functions
+- [itrs](/mod/itrs): Iterator utilities
 - [os](/mod/os): Operating system functions 

_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.