Add reference C code that I can hand compile

reference/gc.h

@@ -0,0 +1,28 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+#define SIZE 8
+#define BROKEN_HEART 1
+#define CONS 2
+#define INTEGER 3
+
+typedef struct box_t {
+  char type;
+  union {
+    int integer;
+    struct cons_t* cons;
+  };
+} box_t;
+
+typedef struct cons_t {
+  box_t car;
+  box_t cdr;
+} cons_t;
+
+void init();
+cons_t *alloc();
+void gc_run();
+void gc_loop();
+void relocate(cons_t*);