Add 'again' macro for recursive expansion

The 'again' macro is a new addition to the macro system, allowing for
recursive macro expansion. This commit introduces the necessary AST
nodes, macro expansion logic, and an integration test to ensure its
functionality. The `soa_series.myc` example has also been updated to
demonstrate its usage.

examples/repeat.myc

@@ -0,0 +1,3 @@
+(do
+  (repeat n 10 (print n)
+)

examples/soa_series.myc

@@ -1,4 +1,4 @@
-;; Output: 26.7
+;; Output: 228
 ;; Benchmark: 1.4us
 ;; Benchmark-Repeat: 1406
 (do
@@ -8,6 +8,8 @@
   (push my_ticks {:price 11.2 :volume 200 :msg "B"})
   (push my_ticks {:price 15.5 :volume 300 :msg "C"})
 
+  (repeat n 100 (push my_ticks {:price (+ n 15.5) :volume (ceil (/ n 300)) :msg "??"})
+)
 
   (def prices (.price my_ticks))
 

src/ast/compiler/macros.rs

@@ -275,6 +275,17 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
+            UntypedKind::Again { args } => {
+                let expanded_args = self.expand_recursive(*args)?;
+                Ok(Node {
+                    identity: node.identity,
+                    kind: UntypedKind::Again {
+                        args: Box::new(expanded_args),
+                    },
+                    ty: (),
+                })
+            }
+
             _ => Ok(node),
         }
     }
@@ -536,6 +547,17 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
+            UntypedKind::Again { args } => {
+                let expanded_args = self.expand_template(*args, state)?;
+                Ok(Node {
+                    identity: node.identity,
+                    kind: UntypedKind::Again {
+                        args: Box::new(expanded_args),
+                    },
+                    ty: (),
+                })
+            }
+
             _ => Ok(node),
         }
     }

src/integration_test.rs

@@ -813,4 +813,20 @@ mod tests {
         let res = env.run_script(&source_fixed);
         assert!(res.is_ok(), "Inlining slot clash triggered: {:?}", res.err());
     }
+
+    #[test]
+    fn test_repro_placeholder_in_template_failure() {
+        let env = Environment::new();
+        let source = r#"
+            (do
+                (macro repeat [var limit body]
+                    `((fn [~var __limit]
+                        (if (< ~var __limit)
+                            (do ~body (again (+ ~var 1) __limit))))
+                      0 ~limit))
+                (repeat n 10 42))
+        "#;
+        let res = env.run_script(source);
+        assert!(res.is_ok(), "Expansion failed to strip placeholders: {:?}", res.err());
+    }
 }