[DateTime] Removed outdated methods

- Instant::atTimeZone() : use ZonedDateTime::ofInstant()
- Instant::fromDateTime() : useless
- Instant::toDateTime() : useless
- ZonedDateTime::ofEpochSecond() : use ZonedDateTime::ofInstant()

ZonedDateTime::ofInstant() now has support for nanoseconds.
[DateTime] Added tests for Instant
[DateTime] Marked utility classes as internal
[DateTime] Refactored tests
[DateTime] Removed ReadableInstant::isBetweenInclusive() and isBetweenExclusive()
[DateTime] Fixed documentation copy/paste typo
[DateTime] Added tests for ReadableInstant