[time] Cleanup exposition-only names

Editorial notes:
  * Apply `\exposid` to all exposition-only members in [time].
  * For _`local-time-format-t`_ and _`STATICALLY-WIDEN`_, change uses of
    `\placeholder` to `\exposid`.
  * Rename some members of `hh_mm_ss`, `zoned_time`, and
    _`local-time-format-t`_ in the style of either _`name_`_ or
    _`kebab-case`_.

Author: frederick-vs-ja