make memoffset advisory informational

[RalfJung]

To cause the drop of uninit memory clients had to panic in a Deref implementation. That sounds like a sufficiently "uncommon" library usage to me that this probably should be just informational.

[tarcieri]

We need to update the rustsec-admin linter to allow unsound as an informational advisory type. Let me do that and cut another release (we've needed to do that for the new assign-id command anyway)

[tarcieri]

@RalfJung can you rebase so it pulls in the new linter? /cc @alex

[RalfJung]

Sure, done.