rust: time: Introduce Instant type

Introduce a type representing a specific point in time. We could use
the Ktime type but C's ktime_t is used for both timestamp and
timedelta. To avoid confusion, introduce a new Instant type for
timestamp.

Rename Ktime to Instant and modify their methods for timestamp.

Implement the subtraction operator for Instant:

Delta = Instant A - Instant B

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>