Feature request: zip as standalone operation

[radujica]

As hinted by grizzly's groupby TODO, using zip(group_on, zip(col1, col2)) would be nice, however that is not supported, i.e. the following is returned:

Unsupported expression: zip(col1, col2)

This forces one to do:

let columns = result(
        for(
            zip(col1, col2),
            appender,
            |b, i, e| merge(b, e)
        )
);

for(zip(group_on, columns),...`

Perhaps there are some performance issues why this was not implemented yet?