Reintroduce alloc_error_handler as deprecated

[Manishearth]

See #109507 (comment): The removal of #[alloc_error_handler] introduces a hard edge for people doing embedded development: code must contain the handler before that PR, and code must not contain that handler after that PR.

While it is an unstable feature and we can remove it immediately, it would be nicer to our embedded users for us to provide a slightly more graceful migration path for a couple releases.

This is not as graceful as it could be, as even with this change code that wants to have a different alloc error handler from its panic handler is still forced to pick a rust version, but at least the code compiles, and for the majority of cases where panics and allocs are going to be handled the same way it doesn't matter.

Not sure of this implementation; I could make it a future incompat lint instead of just using #[deprecated] given that it's a behavior change, not just a deprecation.

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r? @davidtwco

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[Amanieu]

default_alloc_error_handler has been stabilized for a while (#102318) which removes the requirement for no_std users to implement their own #[alloc_error_handler]. handle_alloc_error was already defaulting to calling the panic handler if #[alloc_error_handler] is absent.

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fmt check
Diff in /checkout/library/core/src/prelude/v1.rs at line 77:
 // (no public module for them to be re-exported from).
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 #[allow(deprecated)]
-pub use crate::macros::builtin::{alloc_error_handler, bench, derive, global_allocator, test, test_case};
+pub use crate::macros::builtin::{
+    alloc_error_handler, bench, derive, global_allocator, test, test_case,
 
 #[unstable(feature = "derive_const", issue = "none")]
 pub use crate::macros::builtin::derive_const;
Build completed unsuccessfully in 0:00:23
Build completed unsuccessfully in 0:00:23
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/rustfmt/bin/rustfmt" "--config-path" "/checkout" "--edition" "2021" "--unstable-features" "--skip-children" "--check" "/checkout/library/core/src/mem/manually_drop.rs" "/checkout/library/core/benches/fmt.rs" "/checkout/library/core/src/mem/mod.rs" "/checkout/library/core/src/mem/transmutability.rs" "/checkout/library/core/src/default.rs" "/checkout/library/core/benches/ascii/is_ascii.rs" "/checkout/library/core/src/prelude/v1.rs" "/checkout/library/core/src/mem/maybe_uninit.rs"` failed.
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[Manishearth]

Aha, I see. Thanks! I noticed it didn't work with our pinned nightly (we test embedded stuff with a pinned nightly as well as rolling nightlies) but I misread the date and didn't realize our pinned nightly was a year old. Thanks!