Rollup merge of #60656 - petertodd:2019-inline-cursor-over-slice, r=sfackler

Inline some Cursor calls for slices

(Partially) brings back https://github.com/rust-lang/rust/pull/33921

I've noticed in some serialization code I was writing that writes to slices produce much, much, worse code than you'd expect even with optimizations turned on. For example, you'd expect something like this to be zero cost:

```
use std::io::{self, Cursor, Write};

pub fn serialize((a, b): (u64, u64)) -> [u8;8+8] {
    let mut r = [0u8;16];
    {
        let mut w = Cursor::new(&mut r[..]);

        w.write(&a.to_le_bytes()).unwrap();
        w.write(&b.to_le_bytes()).unwrap();
    }
    r
}
```

...but it compiles down to [dozens of instructions](https://rust.godbolt.org/z/bdwDzb) because the `slice_write()` calls aren't inlined, which in turn means `unwrap()` can't be optimized away, and so on.

To be clear, this pull-req isn't sufficient by itself: if we want to go down that path we also need to add `#[inline]`'s to the default implementations for functions like `write_all()` in the `Write` trait and so on, or implement them separately in the `Cursor` impls. But I figured I'd start a conversation about what tradeoffs we're expecting here.

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