apply #[optimize(size)] to #[cold] ones and part of the panick machinery

library/core/src/lib.rs

@@ -233,6 +233,7 @@
 #![feature(never_type)]
 #![feature(no_core)]
 #![feature(no_sanitize)]
+#![feature(optimize_attribute)]
 #![feature(prelude_import)]
 #![feature(repr_simd)]
 #![feature(rustc_allow_const_fn_unstable)]