addition: size, is empty and capacity

src/arraylist.c

@@ -58,3 +58,24 @@ ArrayListErr arraylist_clear(ArrayList *arr) {
     arr->len = 0;
     return ARRLIST_OK;
 }
+
+size_t arraylist_size(ArrayList *arr) {
+    if (arr == NULL) {
+        return 0;
+    }
+    return arr->len;
+}
+
+size_t arraylist_capacity(ArrayList *arr) {
+    if (arr == NULL) {
+        return 0;
+    }
+    return arr->capacity;
+}
+
+bool arraylist_is_empty(ArrayList *arr) {
+    if (arr == NULL) {
+        return true;
+    }
+    return arr->capacity == 0;
+}