fix #10 set

+collection functions (map, forEach, toList, toSet, isEmpty, etc,)

docs/Collection.md

@@ -0,0 +1,13 @@
+# Collection
+
+Is a [Iterable] with known `size`, a finite [Iterable]:
+
+    class Collection : Iterable {
+        val size
+    }
+
+See [List], [Set] and [Iterable]
+
+[Iterable]: Iterable.md
+[List]: List.md
+[Set]: Set.md