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))