Rollup merge of #143925 - oli-obk:slice-const-partialeq, r=fee1-dead

Make slice comparisons const

This needed a fix for `derive_const`, too, as it wasn't usable in libcore anymore as trait impls need const stability attributes. I think we can't use the same system as normal trait impls while `const_trait_impl` is still unstable.

r? ```@fee1-dead```

cc rust-lang/rust#143800