Cherry-pick std::error downcasting to beta #25121
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This commit brings the `Error` trait in line with the [Error interoperation RFC](rust-lang/rfcs#201) by adding downcasting, which has long been intended. This change means that for any `Error` trait objects that are `'static`, you can downcast to concrete error types. To make this work, it is necessary for `Error` to inherit from `Reflect` (which is currently used to mark concrete types as "permitted for reflection, aka downcasting"). This is a breaking change: it means that impls like ```rust impl<T> Error for MyErrorType<T> { ... } ``` must change to something like ```rust impl<T: Reflect> Error for MyErrorType<T> { ... } ``` except that `Reflect` is currently unstable (and should remain so for the time being). For now, code can instead bound by `Any`: ```rust impl<T: Any> Error for MyErrorType<T> { ... } ``` which *is* stable and has `Reflect` as a super trait. The downside is that this imposes a `'static` constraint, but that only constrains *when* `Error` is implemented -- it does not actually constrain the types that can implement `Error`. [breaking-change] Conflicts: src/libcore/marker.rs
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brson
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Cherry-pick std::error downcasting to beta
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This is a cherry-pick of PR #24793