Use base64 from simdutf8

src/arena_allocator.hpp

@@ -227,7 +227,8 @@ public:
    * @param ptr Pointer to the existing allocation (must be from this allocator)
    * @param old_size Size of the existing allocation in bytes
    * @param new_size Desired new size in bytes
-   * @param alignment Required alignment
+   * @param alignment Required alignment. Defaults to
+   * `alignof(std::max_align_t)`
    * @return Pointer to the reallocated memory (may be the same as ptr or
    * different)
    * @throws std::bad_alloc if memory allocation fails
@@ -253,7 +254,7 @@ public:
    * - When copying to new location, uses the specified alignment
    */
   void *realloc_raw(void *ptr, uint32_t old_size, uint32_t new_size,
-                    uint32_t alignment);
+                    uint32_t alignment = alignof(std::max_align_t));
 
   /**
    * @brief Type-safe version of realloc_raw for arrays of type T.