Enable validation everywhere

[RalfJung]

Validation is currently disabled in a few places, they should all get fixed:

• tests/run-pass/ref-invalid-ptr.rs because we need something like RFC for an operator to take a raw reference rfcs#2582 invalid, now that we require even raw ptr derefs not to dangle
• direct_mut_to_const_raw in tests/run-pass/stacked-borrows.rs waits for a fix for Coercing &mut to *const should not create a shared reference rust#56604
• tests/run-pass/btreemap.rs test, because of BTreeSet causes UB by having (partially) dangling shared reference rust#54957
• cargo miri test test (in Fix cargo miri test #550), because of BTreeSet causes UB by having (partially) dangling shared reference rust#54957
• On all run-pass tests and in the README example when running without full MIR, to be fixed by always requiring full MIR
• run-pass-fullmir/vecdeque.rs waits for VecDeque: fix for stacked borrows rust#56161 to land
• tests/run-pass-fullmir/async-fn.rs waits for fix futures creating aliasing mutable and shared ref rust#56319 to land

We also have a whitelist disabling it for a few functions:

• Some raw pointer operations, will be fixed by std::ptr no longer needs whitelisting #547 (remember to update Fix cargo miri test #550 which adds another one here!)
• Windows' Mutex, can be fixed once use MaybeUninit instead of mem::uninitialized for Windows Mutex rust#56275 lands

[RalfJung]

I think we can close this; the one remaining problem is &mut -> *const, which is tracked at rust-lang/rust#56604 and rust-lang/unsafe-code-guidelines#227.