SimpleTimeline interface polishing about time-steward

[elidupree]

Conclusions:

• Don't wrap the data in Option. Also don't have an initial value. If you query it before the first change, panic.
• Although it's easiest to store every EventHandle, it won't report on them, in the interest of future-proofing the API. (A future implementation might not always have them available.)
• Due to my decisions on #32, it will have an optional "destruction time", and at least debug_assert that you can't query it after the destruction time.
• It's not currently worthwhile to make additional features to work around the lack of query-by-reference.
• Since modify_simple_timeline() is already in the simple_timeline module, rename it to set() and unmodify_simple_timeline() to unset().
• It's not currently worthwhile to make several variants of SimpleTimeline. If we make variants in the future, it might involve renaming the current struct, but that's relatively easy to fix in the examples (it would be a simple find-and-replace, or a type alias, or use Foo as Bar).