This is similar to `NonNull::cast`.

Compared to the `as` operator (which has a wide range of meanings
depending on the input and output types), a call to this method:

* Can only go from a raw pointer to a raw pointer
* Cannot change the pointer’s `const`ness

… even when the pointed types are inferred based on context.

(Partially) brings back rust-lang#33921

Add a `cast` method to raw pointers.

This is similar to `NonNull::cast`.

Compared to the `as` operator (which has a wide range of meanings depending on the input and output types), a call to this method:

* Can only go from a raw pointer to a raw pointer
* Cannot change the pointer’s `const`ness

… even when the pointed types are inferred based on context.

pin: make the to-module link more visible

Cc @gnzlbg

…ister

cleanup: Remove `DefIndexAddressSpace`

The scheme with two address spaces for `DefIndex` was needed in the past, but apparently not needed anymore (after removing `DefId`s from locals and `HirId`-ification).

…lice, r=sfackler

Inline some Cursor calls for slices

(Partially) brings back rust-lang#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.

…apin

Stabilize and re-export core::array in std

Fixes rust-lang#60014