Auto merge of #95770 - nrc:read-buf-builder, r=joshtriplett

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 https://github.com/rust-lang/rust/issues/78485, https://github.com/rust-lang/rust/issues/94741

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