test and rework: Added push test and cahnged arena_push return

So added a little test that just adds 3 ints to an arena, nothing much
but the basics work, also changed arena_push because it only pushed and
didn't returned a ponter (wich is fucking useless) so also fixed that,
everything seems fine now, need more tests.

# Tipos:
# feat, fix, refactor, docs, style, test, chore

src/arena.c

@@ -82,27 +82,39 @@ ArenaPointer arena_alloc(Arena *arena, size_t size, size_t alignment) {
     return val;
 }
 
-ArenaErr arena_push(Arena *arena, void *data, size_t size, size_t alignment) {
+ArenaPointer arena_push(Arena *arena, void *data, size_t size, size_t alignment) {
     if (arena == NULL || data == NULL) {
-        return ARENA_NULL_ARG;
+        ArenaPointer bad_pointer = {
+            .is_valid = false,
+            .err = ARENA_NULL_ARG,
+        };
+        return bad_pointer;
     }
 
     ArenaPointer pointer = arena_alloc(arena, size, alignment);
 
     if (!pointer.is_valid) {
-        return pointer.err;
+        return pointer;
     }
     while (!pointer.is_valid && pointer.err == ARENA_OUT_OF_SPACE) {
         size_t new_capacity;
 
         bool is_overflow = mul_size_t_safe(arena->capacity, 2, &new_capacity);
         if (is_overflow) {
-            return ARENA_CAPACITY_OVERFLOW;
+            ArenaPointer bad_pointer = {
+                .is_valid = false,
+                .err = ARENA_CAPACITY_OVERFLOW,
+            };
+            return bad_pointer;
         }
 
         ArenaErr err = arena_realloc(arena, new_capacity);
         if (err != ARENA_OK) {
-            return err;
+            ArenaPointer bad_pointer = {
+                .is_valid = false,
+                .err = err,
+            };
+            return bad_pointer;
         }
 
         pointer = arena_alloc(arena, size, alignment);
@@ -113,7 +125,7 @@ ArenaErr arena_push(Arena *arena, void *data, size_t size, size_t alignment) {
         data,
         size);
 
-    return ARENA_OK;
+    return pointer;
 }
 
 ArenaErr arena_realloc(Arena *arena, size_t new_capacity) {