Attach and other fixes

[xitology]

• undefine(): preserve group type in order not to block aggregates
• group_with(): add trailing undefine(name) to prohibit direct access to the attached row
• implement attach_earliest() and attach_latest(), similar to filter_with() and group_with()

@funsql begin
    person()
    attach_earliest(
        earliest_bmi => measurement(LOINC("39156-5", "Body mass index (BMI) [Ratio]")))
    attach_latest(
        latest_bmi => measurement(LOINC("39156-5", "Body mass index (BMI) [Ratio]")))
    select(
        person_id,
        earliest_bmi_datetime => earliest_bmi.datetime,
        earliest_bmi_value => earliest_bmi.value_as_number,
        latest_bmi_datetime => latest_bmi.datetime,
        latest_bmi_value => latest_bmi.value_as_number)
end

Just like group_with() and filter_with(), they also accept an optional predicate parameter.