Change `startOfDay` and `trimNanoSec` to use `initDateTime` instead of
mutating the new copy via the deprecated property accessors.
Updated the test suite to reflect functions that moved from this library
to the standard library. We're still testing the expected functionality
to make sure that the contract is maintained for users of this library.
* `startOfDay(TimeInfo): TimeInfo` returns a new `TimeInfo` representing
midnight at the beginning of the given day.
* `startOfWeek(TimeInfo, WeekDay): TimeInfo` returns a new `TimeInfo`
representing midnight at the beginning of the first day of the week. By
default Monday is used as the start of the week (to be consistent with
`times` view of the day order), but the user can pass in any other day to
"start" the week. Because this find the start of the *current* week, the
returned `TimeInfo` will always be a date in the past or present, never a
date in the future.
