Rollup merge of #94657 - fee1-dead:const_slice_index, r=oli-obk

Constify `Index{,Mut}` for `[T]`, `str`, and `[T; N]`

Several panic functions were rewired (via `const_eval_select`) to simpler implementations that do not require formatting for compile-time usage.

r? ```@oli-obk```

library/core/src/lib.rs

@@ -149,6 +149,8 @@
 #![feature(variant_count)]
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]
+#![feature(const_slice_index)]
+#![feature(const_is_char_boundary)]
 //
 // Language features:
 #![feature(abi_unadjusted)]