Rollup merge of #74707 - matklad:split_once, r=dtolnay

Add str::[r]split_once

This is useful for quick&dirty parsing of key: value config pairs. Used a bunch in Cargo and rust-analyzer:

* https://github.com/rust-lang/cargo/search?q=splitn%282&unscoped_q=splitn%282
* https://github.com/rust-analyzer/rust-analyzer/search?q=split_delim&unscoped_q=split_delim

In theory, once const-generics are done, this functionality could be achieved without a dedicated method with

```rust
match s.splitn(delimier, 2).collect_array::<2>() {
  Some([prefix, suffix]) => todo!(),
  None => todo!(),
}
```

Even in that world, having a dedicated method seems clearer on the intention.

I am not sure about naming -- this is something I've just came up with yesterday, I don't know off the top of my head analogs in other languages.

If T-libs thinks this is a reasonable API to have, I'll open a tracking issue and add more thorough tests.

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