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

if (col_in->getN() != IPV6_BINARY_LENGTH)
                throw Exception("Illegal type " + col_type_name.type->getName() +
                                " of column " + col_in->getName() +
                                " argument of function " + getName() +
                                ", expected FixedString(" + toString(IPV6_BINARY_LENGTH) + ")",
                                ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
            const auto size = col_in->size();
            const auto & vec_in = col_in->getChars();
            auto col_res = ColumnString::create();
            ColumnString::Chars & vec_res = col_res->getChars();
            ColumnString::Offsets & offsets_res = col_res->getOffsets();
            vec_res.resize(size * (IPV6_MAX_TEXT_LENGTH + 1));
            offsets_res.resize(size);
            auto * begin = reinterpret_cast<char *>(vec_res.data());
            auto * pos = begin;
            for (size_t offset = 0, i = 0; offset < vec_in.size(); offset += IPV6_BINARY_LENGTH, ++i)
            {
                formatIPv6(reinterpret_cast<const unsigned char *>(&vec_in[offset]), pos);
                offsets_res[i] = pos - begin;
            }
            vec_res.resize(pos - begin);
            return col_res;
        }
        else
            throw Exception("Illegal column " + arguments[0].column->getName()
                + " of argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN);
    }
};
class FunctionCutIPv6 : public IFunction