rust: time: Implement Add<Delta>/Sub<Delta> for Instant

rust: time: Implement Add<Delta>/Sub<Delta> for Instant

In order to copy the behavior rust currently follows for basic arithmetic
operations and panic if the result of an addition or subtraction results in
a value that would violate the invariants of Instant, but only if the
kernel has overflow checking for rust enabled.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250820203704.731588-2-lyude@redhat.com
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>