Replace Unique in Box with a (NonNull, PhantomData) wrapper

[maxdexh]

Follow-up to #162804

See zulip.

This is only the first step in actually refactoring Box, and is essentially just a rename.
The layout of Box stays as-is for now, due to #162850.

Needs perf run

[rustbot]

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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[maxdexh]

@hanna-kruppe do you want to take this one too?

r? hanna-kruppe

Reroll libs if not, please (sorry, should have asked before marking the PR as ready)

[maxdexh]

ignore rustbot, all changes outside libs were to test files ^^

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