**Use case**

Convert bitmask to position.
It would allow us to directly use FLAT dictionary or Enum datatype without jumping around slow `bitmasktoArray` and `arrayMap`

**Describe the solution you'd like**

```
SELECT
    arrayMap(x -> log2(x), bitmaskToArray(100)) AS bitmask,
    bitPositions(100)  AS bitpositions

┌─bitmask─┬─bitpositions─┐
│ [2,5,6] │ [2,5,6]      │
└─────────┴──────────────┘
```

{
public:
    static constexpr auto name = "IPv6NumToString";
    static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionIPv6NumToString>(); }
    String getName() const override { return name; }
    size_t getNumberOfArguments() const override { return 1; }
    bool isInjective(const ColumnsWithTypeAndName &) const override { return true; }
    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        const auto * ptr = checkAndGetDataType<DataTypeFixedString>(arguments[0].get());
        if (!ptr || ptr->getN() != IPV6_BINARY_LENGTH)
            throw Exception("Illegal type " + arguments[0]->getName() +
                            " of argument of function " + getName() +
                            ", expected FixedString(" + toString(IPV6_BINARY_LENGTH) + ")",
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
        return std::make_shared<DataTypeString>();
    }
    bool useDefaultImplementationForConstants() const override { return true; }
    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t ) const override
    {
        const auto & col_type_name = arguments[0];
        const ColumnPtr & column = col_type_name.column;
        if (const auto * col_in = checkAndGetColumn<ColumnFixedString>(column.get()))
        {