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

bool isInjective(const ColumnsWithTypeAndName &) const override { return true; }
    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        if (!checkAndGetDataType<DataTypeUInt32>(arguments[0].get()))
            throw Exception("Illegal type " + arguments[0]->getName() +
                            " of argument of function " + getName(), ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
        return std::make_shared<DataTypeFixedString>(16);
    }
    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 = typeid_cast<const ColumnUInt32 *>(column.get()))
        {
            auto col_res = ColumnFixedString::create(IPV6_BINARY_LENGTH);
            auto & vec_res = col_res->getChars();
            vec_res.resize(col_in->size() * IPV6_BINARY_LENGTH);
            const auto & vec_in = col_in->getData();
            for (size_t out_offset = 0, i = 0; out_offset < vec_res.size(); out_offset += IPV6_BINARY_LENGTH, ++i)
                mapIPv4ToIPv6(vec_in[i], &vec_res[out_offset]);
            return col_res;
        }
        else
            throw Exception("Illegal column " + arguments[0].column->getName()
                + " of argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN);
    }
private:
    static void mapIPv4ToIPv6(UInt32 in, UInt8 * buf)