Auto merge of #89582 - jkugelman:optimize-file-read-to-end, r=joshtriplett

Auto merge of #89582 - jkugelman:optimize-file-read-to-end, r=joshtriplett

Optimize File::read_to_end and read_to_string

Reading a file into an empty vector or string buffer can incur unnecessary `read` syscalls and memory re-allocations as the buffer "warms up" and grows to its final size. This is perhaps a necessary evil with generic readers, but files can be read in smarter by checking the file size and reserving that much capacity.

`std::fs::read` and `std::fs::read_to_string` already perform this optimization: they open the file, reads its metadata, and call `with_capacity` with the file size. This ensures that the buffer does not need to be resized and an initial string of small `read` syscalls.

However, if a user opens the `File` themselves and calls `file.read_to_end` or `file.read_to_string` they do not get this optimization.

```rust
let mut buf = Vec::new();
file.read_to_end(&mut buf)?;
```

I searched through this project's codebase and even here are a *lot* of examples of this. They're found all over in unit tests, which isn't a big deal, but there are also several real instances in the compiler and in Cargo. I've documented the ones I found in a comment here:

https://github.com/rust-lang/rust/issues/89516#issuecomment-934423999

Most telling, the documentation for both the `Read` trait and the `Read::read_to_end` method both show this exact pattern as examples of how to use readers. What this says to me is that this shouldn't be solved by simply fixing the instances of it in this codebase. If it's here it's certain to be prevalent in the wider Rust ecosystem.

To that end, this commit adds specializations of `read_to_end` and `read_to_string` directly on `File`. This way it's no longer a minor footgun to start with an empty buffer when reading a file in.

A nice side effect of this change is that code that accesses a `File` as `impl Read` or `dyn Read` will benefit. For example, this code from `compiler/rustc_serialize/src/json.rs`:

```rust
pub fn from_reader(rdr: &mut dyn Read) -> Result<Json, BuilderError> {
    let mut contents = Vec::new();
    match rdr.read_to_end(&mut contents) {
```

Related changes:

- I also added specializations to `BufReader` to delegate to `self.inner`'s methods. That way it can call `File`'s optimized  implementations if the inner reader is a file.

- The private `std::io::append_to_string` function is now marked `unsafe`.

- `File::read_to_string` being more efficient means that the performance note for `io::read_to_string` can be softened. I've added `@camelid's` suggested wording from https://github.com/rust-lang/rust/issues/80218#issuecomment-936806502.

r? `@joshtriplett`