test, add regression cases for bug #24
minor, introduce dt_separable_p() to denote a predicate ...

that indicates whether or not it's possible to access the
slots .d and .t separately.
minor, separate sexy to daisy/hms sandwich and ymdhms to ymd/hms sandwich
hygiene, use dt_dt_s struct TYP values in dt_separable_p()
Merge branch 'bug/24'

* bug/24:
  hygiene, use dt_dt_s struct TYP values in dt_separable_p()
  minor, separate sexy to daisy/hms sandwich and ymdhms to ymd/hms sandwich
  minor, introduce dt_separable_p() to denote a predicate ...
  test, add regression cases for bug #24
fix, PEBKAC, convert sexy/ymdhms pack according to tgttyp
Merge branch 'bug/ddiff-ymd+sexy'

* bug/ddiff-ymd+sexy:
  fix, PEBKAC, convert sexy/ymdhms pack according to tgttyp