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.

include/arena.h

@@ -6,11 +6,7 @@
 #include <inttypes.h>
 #include <stdbool.h>
 
-typedef struct {
-    uint8_t *buffer;
-    size_t capacity;
-    size_t offset;
-} Arena;
+typedef struct Arena Arena;
 
 typedef enum {
     ARENA_OK = 0,
@@ -37,7 +33,7 @@ typedef struct {
     bool is_valid;
     union {
         ArenaErr err;
-        Arena arena;
+        Arena *arena;
     };
 } ArenaResult;
 
@@ -49,8 +45,8 @@ typedef struct {
     };
 } ArenaSizeResult;
 
-ArenaResult arena_init(size_t capacity);
-void arena_destroy(Arena *arena);
+ArenaErr arena_init(Arena **arena, size_t capacity);
+void arena_destroy(Arena **arena);
 
 ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment);
 ArenaPointer arena_push(Arena *arena, void *data, size_t size, size_t alignment);

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) {