refactor: Modified ArenaPointer

ArenaPointer now contains a reference to the original arena and an
offset instad of a raw address. Change was made to avoid invalidating
addresses and keeping a more consise system. arena_unwrap_pointer was
also included as a helper function.

include/arena.h

@@ -26,7 +26,10 @@ typedef struct {
     bool is_valid;
     union {
         ArenaErr err;
-        uint8_t *address;
+        struct {
+            Arena *arena;
+            size_t offset;
+        };
     };
 } ArenaPointer;
 
@@ -53,6 +56,8 @@ ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment);
 ArenaPointer arena_push(Arena *arena, void *data, size_t size, size_t alignment);
 ArenaErr arena_realloc(Arena *arena, size_t new_capacity);
 
+void *arena_unwrap_pointer(ArenaPointer p);
+
 SizeResult get_arena_align_padding(Arena *arena, size_t alignment);
 ArenaErr arena_ensure_capacity(Arena *arena, size_t size, size_t alignment);
 // Should be moved to something like general utilities,
@@ -60,7 +65,4 @@ ArenaErr arena_ensure_capacity(Arena *arena, size_t size, size_t alignment);
 bool mul_size_t_safe(size_t a, size_t b, size_t *out);
 
 
-
-
-
 #endif // !ARENA_H