Auto merge of #45837 - SimonSapin:file_read_write, r=dtolnay

Add read, read_string, and write functions to std::fs

New APIs in `std::fs`:

```rust
pub fn read<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> { … }
pub fn read_string<P: AsRef<Path>>(path: P) -> io::Result<String> { … }
pub fn write<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> io::Result<()> { ... }
```

(`read_string` is based on `read_to_string` and so returns an error on non-UTF-8 content.)

Before:

```rust
use std::fs::File;
use std::io::Read;

let mut bytes = Vec::new();
File::open(filename)?.read_to_end(&mut bytes)?;
do_something_with(bytes)
```

After:

```rust
use std::fs;

do_something_with(fs::read(filename)?)
```

0 files changed, 0 insertions, 0 deletions