Auto merge of #53619 - japaric:panic-handler, r=SimonSapin

add #[panic_handler]; deprecate #[panic_implementation]

r? @SimonSapin
cc #44489

diff --cc src/librustc_typeck/check/mod.rs
Simple merge

diff --cc src/libstd/lib.rs
Simple merge

diff --cc src/libsyntax/feature_gate.rs
Simple merge