Implement length for all enumerables

[davidanthoff]

This example doesn't work without a collect for the sub-query currently, and that is annoying:

rts = @from row in results begin
    @where row.trial >= 1
    @group row by (row.sid, row.trial) into trial
    @let filtered_trial = @where(trial, i -> i.code in ["stream_1", "stream_2"])
    @where length(filtered_trial)>0
    @select {
        rt = last(filtered_trial).time - first(filtered_trial).time,
        response = last(filtered_trial).code,
        trial_length = last(trial).time - first(trial).time
    }
    @collect DataFrame
end