Rust changes that have caused ICU4X problems

[sffc]

This is a record I'm keeping of changes made by the Rust team that have broken the ICU4X team.

• When they changed semantics on DSTs and derives with repr(packed) Compile error with recent nightly toolchain unicode-org/icu4x#3060
• When they broke nightly users of #[alloc_error_handler] Remove use of alloc_error_handler, update nightly to 2023-01-01, update tiny test to LLVM 15 unicode-org/icu4x#3382 (seems that they had a transition window, but it wasn't a very long one, and there were no warnings until it broke one day)
• When they broke wasm-wasi with a narrow transition window that wasn't suitable for clients with specific LLVM constraints Fix nightly WASM target unicode-org/icu4x#5799
• When they changed repr(packed) to imply repr(Rust,packed) instead of repr(C,packed) despite the web reality being that repr(packed) had been C-like Panic while formatting date in beta and nightly compilers unicode-org/icu4x#5039
• When the broke WASM ABI Rust Wasm ABI is unstable and will be changed eventually rust-diplomat/diplomat#661
• When they changed the unused_crate_dependencies lint False positive regression for unused_crate_dependencies rust-lang/rust#147069

(I intend to edit this comment to keep it up-to-date as more issues arise)