This allows using `ReadBuf` in a builder-like style and to setup a `ReadBuf` and
pass it to `read_buf` in a single expression, e.g.,

```
// With this PR:
reader.read_buf(ReadBuf::uninit(buf).assume_init(init_len))?;

// Previously:
let mut buf = ReadBuf::uninit(buf);
buf.assume_init(init_len);
reader.read_buf(&mut buf)?;
```

Signed-off-by: Nick Cameron <nrc@ncameron.org>

std::io: Modify some ReadBuf method signatures to return `&mut Self`

This allows using `ReadBuf` in a builder-like style and to setup a `ReadBuf` and
pass it to `read_buf` in a single expression, e.g.,

```
// With this PR:
reader.read_buf(ReadBuf::uninit(buf).assume_init(init_len))?;

// Previously:
let mut buf = ReadBuf::uninit(buf);
buf.assume_init(init_len);
reader.read_buf(&mut buf)?;
```

r? `@sfackler`

cc rust-lang#78485, rust-lang#94741

…nTitor,yaahc

update docs for `std::future::IntoFuture`

Ref rust-lang#67644.

This updates the docs for `IntoFuture` providing a bit more guidance on how to use it. Thanks!

…serve, r=the8472

Remove redundant calls to reserve in impl Write for VecDeque

Removes the reserve calls made redundant by rust-lang#95904 (as discussed in rust-lang#95632 (comment))