Rewrite everything

include/arena.h

@@ -4,16 +4,17 @@
 
 #include <stddef.h> // ptrdiff_t
 
+typedef unsigned char byte_t;
+
 typedef struct arena {
-    void *begin,
-        *next,
-        *prev;
-    size_t cap;
+    size_t next, prev, cap;
+    void* data;
 } __attribute__((aligned(sizeof(void*)))) arena_t;
 
-arena_t* arena_new();
+arena_t arena_new();
 void arena_reset(arena_t* a);
 void* arena_alloc(arena_t* a, size_t len);
+void* arena_calloc(arena_t* a, size_t nmemb, size_t size);
 void* arena_realloc_tail(arena_t* a, size_t len);
 
 #endif