**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]      │
└─────────┴──────────────┘
```

String getName() const override { return name; }
    size_t getNumberOfArguments() const override { return 2; }
    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        const auto * first_argument = checkAndGetDataType<DataTypeFixedString>(arguments[0].get());
        if (!first_argument || first_argument->getN() != IPV6_BINARY_LENGTH)
            throw Exception("Illegal type " + arguments[0]->getName() +
                            " of first argument of function " + getName() +
                            ", expected FixedString(" + toString(IPV6_BINARY_LENGTH) + ")",
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
        const DataTypePtr & second_argument = arguments[1];
        if (!isUInt8(second_argument))
            throw Exception{"Illegal type " + second_argument->getName()
                            + " of second argument of function " + getName()
                            + ", expected UInt8", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
        DataTypePtr element = DataTypeFactory::instance().get("IPv6");
        return std::make_shared<DataTypeTuple>(DataTypes{element, element});
    }
    bool useDefaultImplementationForConstants() const override { return true; }