Method insertNulls

API/fleece/Mutable.hh:130–130

        void append(T v)                        {append() = v;}

        void insertNulls(uint32_t i, uint32_t n) {FLMutableArray_Insert(*this, i, n);}

        // This enables e.g. `array[10] = 17`
        inline keyref<MutableArray,uint32_t> operator[] (int i) {