Rollup merge of #57350 - folex:master, r=estebank

Better error note on unimplemented Index trait for string

fixes #56740

I've tried to compile suggestion from comments in the issue #56740, but unsure of it. So I'm open to advice :)

Current output will be like this:
```rust
error[E0277]: the type `str` cannot be indexed by `{integer}`
  --> $DIR/str-idx.rs:3:17
   |
LL |     let c: u8 = s[4]; //~ ERROR the type `str` cannot be indexed by `{integer}`
   |                 ^^^^ `str` cannot be indexed by `{integer}`
   |
   = help: the trait `std::ops::Index<{integer}>` is not implemented for `str`
   = note: you can use `.chars().nth()` or `.bytes().nth()`
           see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
```

`x.py test src/test/ui` succeeded and I've also tested output manually by compiling the following code:
```rust
fn _f() {
    let s = std::string::String::from("hello");
    let _c = s[0];

    let s = std::string::String::from("hello");
    let mut _c = s[0];

    let s = "hello";
    let _c = s[0];

    let s = "hello";
    let mut _c = &s[0];
}
```

Not sure if some docs should be changed too. I will also fix error message in the [Book :: Indexing into Strings](https://github.com/rust-lang/book/blob/db53e2e3cdf77beac853df6f29db4b3b86ea598c/src/ch08-02-strings.md#indexing-into-strings) if that PR will get approved :)

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