allow large Size again
This basically reverts most of https://github.com/rust-lang/rust/pull/80042, and instead does the panic in `bits()` with a `#[cold]` function to make sure it does not get inlined.
https://github.com/rust-lang/rust/pull/80042 added a comment about an invariant ("The top 3 bits are ALWAYS zero") that is not actually enforced, and if it were enforced that would be a problem for https://github.com/rust-lang/rust/pull/95388. So I think we should not have that invariant, and I adjusted the code accordingly.
r? `@oli-obk` Cc `@sivadeilra`
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