replace mock allocator with simple k&r malloc

include/picrin.h

@@ -17,11 +17,15 @@ struct pic_proc {
   } u;
 };
 
+#define PIC_HEAP_SIZE 1024
+
 typedef struct {
   pic_value *sp;
   pic_value *stbase, *stend;
 
   struct pic_env *global_env;
+
+  struct heap_page *heap;
 } pic_state;
 
 void *pic_alloc(pic_state *, size_t);

include/picrin/gc.h

@@ -0,0 +1,16 @@
+#ifndef GC_H__
+#define GC_H__
+
+struct header {
+  struct header *ptr;
+  size_t size;
+};
+
+struct heap_page {
+  struct header *base, *freep;
+  size_t heap_size;
+};
+
+void init_heap_page(struct heap_page *);
+
+#endif