simdjson_key_count should properly return sizes larger than 0xFFFFFF(16777215)

[TysonAndre]

If the value of size() is 0xFFFFFF then iterate over the object/array instead to count the keys

Add test cases.

// class simdjson::dom::array 
  /**
   * Get the size of the array (number of immediate children).
   * It is a saturated value with a maximum of 0xFFFFFF: if the value
   * is 0xFFFFFF then the size is 0xFFFFFF or greater.
   */
  inline size_t size() const noexcept;

// class simdjson::dom::object
  /**
   * Get the size of the object (number of keys).
   * It is a saturated value with a maximum of 0xFFFFFF: if the value
   * is 0xFFFFFF then the size is 0xFFFFFF or greater.
   */
  inline size_t size() const noexcept;