Reorganized file structure

src/build/ast/Loop.go

@@ -0,0 +1,12 @@
+package ast
+
+import "fmt"
+
+// Loop represents a block of repeatable statements.
+type Loop struct {
+	Body AST
+}
+
+func (node *Loop) String() string {
+	return fmt.Sprintf("(loop %s)", node.Body)
+}