Use Vec extend instead of repeated pushes on several places

Inspired by rust-lang#90813, I tried to use a simple regex (`for .*in.*\{\n.*push\(.*\);\n\s+}`) to search for more places that would use `Vec::push` in a loop and replace them with `Vec::extend`.

These probably won't have as much perf. impact as the original PR (if any), but it would probably be better to do a perf run to see if there are not any regressions.

Improve 'cannot contain emoji' error.

Before:

```
error: identifiers cannot contain emoji: `🦀`
 --> src/main.rs:2:9
  |
2 |     let 🦀 = 1;
  |         ^^
```

After:
```
error: Ferris cannot be used as an identifier
 --> src/main.rs:2:9
  |
2 |     let 🦀 = 1;
  |         ^^ help: try using their name instead: `ferris`
```

r? `@estebank`

Pretty print break and continue without redundant space

**Repro:**

```rust
macro_rules! m {
    ($e:expr) => { stringify!($e) };
}
fn main() {
    println!("{:?}", m!(loop { break; }));
    println!("{:?}", m!(loop { break 'a; }));
    println!("{:?}", m!(loop { break false; }));
}
```

**Before:**

- `"loop { break ; }"`
- `"loop { break 'a ; }"`
- `"loop { break false ; }"`

**After:**

- `"loop { break; }"`
- `"loop { break 'a; }"`
- `"loop { break false; }"`

<br>

Notice that `return` and `yield` already follow the same approach as this PR of printing the space *before* each additional piece following the keyword, rather than *after* each thing.

https://github.com/rust-lang/rust/blob/772d51f887fa407216860bf8ecf3f1a32fb795b4/compiler/rustc_ast_pretty/src/pprust/state.rs#L2148-L2154

https://github.com/rust-lang/rust/blob/772d51f887fa407216860bf8ecf3f1a32fb795b4/compiler/rustc_ast_pretty/src/pprust/state.rs#L2221-L2228

…lett

Implement `core::future::join!`

`join!` polls multiple futures concurrently and returns their outputs.

```rust
async fn run() {
    let (a, b) = join!(async { 0 }, async { 1 });
}
```

cc `@rust-lang/wg-async-foundations`

update Miri

Fixes rust-lang#91649
r? `@ghost`