- Adding a new virtual method Type_handler::Item_time_precision()
- Adding a new virtual method Type_handler::Item_datetime_precision()
- Removing Item::temporal_precision() and adding Item::time_precision()
  and Item::datetime_precision() instead.
- Moving Item_func_convert_tz::fix_length_and_dec() from item_timefunc.cc
  to item_timefunc.h. It's only two lines, and we're changing it anyway.
- Removing Item_temporal_typecast::fix_length_and_dec_generic(),
  moving this code to
  Type_handler::Item_{date|time|datetime}_typecast_fix_length_and_dec().
  This allows to get rid of one more field_type() call.
  Also, in the old reduction, Item_date_typecast::fix_length_and_dec()
  unnecessarily called args[0]->temporal_precision(). The new reduction
  does not call args[0]->datetime_precision(), as DATE does not
  have fractional digits.