Removed more partly constructed datum_t usage.
Removed some more uses of partially constructed datums.
Made datum_t::drop_literals saner and not use datum_ptr_t while we are at it.
Removed last of datum_t(type_t) constructor users.
Removed now-unused datum_t(type_t) constructor.
Made pathspec unproject use datum_object_builder_t.
Made datum_t::merge use datum_object_builder_t instead of datum_ptr_t.
Migrated datum_t::merge to datum_object_builder_t.
Made time_add use datum_object_builder_t.
Made time_sub use datum_object_builder_t.
Made time_in_tz use datum_object_builder_t.