Fixed a bug where some Int*Value types used larger underlying primiti…

…ve types.

* Int8Value and Int16Value were incorrectly using int32 under the hood. This has
  been fixed.
* UInt8Value was incorrectly using uint16 under the hood. Also fixed.

Added API docs for Range and RangeValue.

Fixed the TimeValue LessThan() test so it's now deterministic.

* Previously we were relying on two calls to time.Now() to return different times. This
  failed in github's action test infra. Maybe they're mocking time.Now() or we just happen
  to run that fast? Who cares. It's fixed.