Add an `Unserializable` type to utils

[emilio]

This allows unifying the way we prevent the serialisation of things that
can't be sent over IPC for their in-process nature.

This is a proposed solution to things like #9373

Where something like the following can be used:

    Exit(Unserializable<Sender<()>>),

[emilio]

r? @jdm or @Ms2ger

[jdm]

Thank you for doing this experiment; it's useful to see what it looks like in practice! I think the result doesn't end up being more readable, however, so I'm inclined to put this aside.

[emilio]

Yeah, I agree, maybe calling it InProcess or similar could make it
way more readable, but I understand if you don't agree with that :P

I think that at some point more use cases for something like this could
arise, to the point it could be a good option to remove multiple
panicking serialize implementations.

While it isn't the case I'm fine with leaving this closed though.

On Tue, Jan 19, 2016 at 02:00:29PM -0800, Josh Matthews wrote:

Thank you for doing this experiment; it's useful to see what it looks like in practice! I think the result doesn't end up being more readable, however, so I'm inclined to put this aside.

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#9379 (comment)

[jdm]

Yes, if there turn out to be more of them in the future we can resurrect this.