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

{
        const ColumnPtr & column = arguments[0].column;
        if (const ColumnString * col = checkAndGetColumn<ColumnString>(column.get()))
        {
            auto col_res = ColumnUInt8::create();
            ColumnUInt8::Container & vec_res = col_res->getData();
            vec_res.resize(col->size());
            const ColumnString::Chars & vec_src = col->getChars();
            const ColumnString::Offsets & offsets_src = col->getOffsets();
            size_t prev_offset = 0;
            char v[IPV6_BINARY_LENGTH];
            for (size_t i = 0; i < vec_res.size(); ++i)
            {
                vec_res[i] = DB::parseIPv6(reinterpret_cast<const char *>(&vec_src[prev_offset]), reinterpret_cast<unsigned char*>(v));
                prev_offset = offsets_src[i];
            }
            return col_res;
        }
        else
            throw Exception("Illegal column " + arguments[0].column->getName()
                            + " of argument of function " + getName(),
                            ErrorCodes::ILLEGAL_COLUMN);
    }
};
}