Implement example iterator

src/spectator/example_iterator.cr

@@ -0,0 +1,42 @@
+module Spectator
+  class ExampleIterator
+    include Iterator(Example)
+
+    @stack : Array(Iterator(ExampleComponent))
+
+    def initialize(@group : Iterable(ExampleComponent))
+      iter = @group.each.as(Iterator(ExampleComponent))
+      @stack = [iter]
+    end
+
+    def next
+      until @stack.empty?
+        item = advance
+        return item if item.is_a?(Example)
+      end
+      stop
+    end
+
+    def rewind
+      iter = @group.each.as(Iterator(ExampleComponent))
+      @stack = [iter]
+      self
+    end
+
+    private def top
+      @stack.last
+    end
+
+    private def advance
+      item = top.next
+      case(item)
+      when ExampleGroup
+        @stack.push(item.each)
+      when Iterator::Stop
+        @stack.pop
+      else
+        item
+      end
+    end
+  end
+end