Pull the guts of `ResponseInstruction` into a new enum

Pull the guts of `ResponseInstruction` into a new enum

In the coming commits we'll use the `ResponseInstruction` enum's
contents for all messages, allowing message senders to rely on the
in-`OnionMessegner` reply path selection logic.

In order to do so and avoid users confusing the new
`MessageSendInstructions` for `ResponseInstruction`, we leave
`ResponseInstruction` as a now-unconstructible struct which wraps a
`MessageSendInstructions`.