refactor: Arena is opaque, init returns error code

Is to make it more consistent, i need to change also later arraylist the
init function so its also more consistent, next is making alloc ensure
capacity first so no using it first or some shit.

src/arena.c

@@ -5,6 +5,11 @@
 #include <stdlib.h>
 #include <string.h>
 
+struct Arena {
+    uint8_t *buffer;
+    size_t capacity;
+    size_t offset;
+};
 
 
 ArenaResult arena_init(size_t capacity) {