Implemented if statements

src/build/ast/AST.go

@@ -37,6 +37,15 @@ func (node *Define) String() string {
 	return fmt.Sprintf("(= %s %s)", node.Name.Text(), node.Value)
 }
 
+type If struct {
+	Condition *expression.Expression
+	Body      AST
+}
+
+func (node *If) String() string {
+	return fmt.Sprintf("(if %s %s)", node.Condition, node.Body)
+}
+
 type Loop struct {
 	Body AST
 }