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```

diff --cc library/core/src/lib.rs
Simple merge

diff --cc library/core/src/ptr/non_null.rs
Simple merge

diff --cc library/core/src/slice/mod.rs
Simple merge