Rollup merge of #97569 - thomcc:fill_with_isnt_memset, r=Amanieu

Rollup merge of #97569 - thomcc:fill_with_isnt_memset, r=Amanieu

Remove `memset` alias from `fill_with`.

In https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Unsafe.20and.20Safe.20versions.20of.20APIs.20both.20getting.20the.20same.20alias/near/284413029 `@Amanieu` pointed out that we had this, which is not really right.

In our guidelines we say that we will "not add an alias for a function that's only somewhat similar or related", which applies here. Memset doesn't take a closure, not even conceptually.