Rollup merge of #128399 - mammothbane:master, r=Amanieu,tgross35

liballoc: introduce String, Vec const-slicing

This change `const`-qualifies many methods on `Vec` and `String`, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice`.

## Motivation
This is to support simultaneous variance over ownership and constness. I have an enum type that may contain either `String` or `&str`, and I want to produce a `&str` from it in a possibly-`const` context.

```rust
enum StrOrString<'s> {
    Str(&'s str),
    String(String),
}

impl<'s> StrOrString<'s> {
    const fn as_str(&self) -> &str {
        match self {
             // In a const-context, I really only expect to see this variant, but I can't switch the implementation
             // in some mode like #[cfg(const)] -- there has to be a single body
             Self::Str(s) => s,

             // so this is a problem, since it's not `const`
             Self::String(s) => s.as_str(),
        }
    }
}
```

Currently `String` and `Vec` don't support this, but can without functional changes. Similar logic applies for `len`, `capacity`, `is_empty`.

## Changes

The essential thing enabling this change is that `Unique::as_ptr` is `const`. This lets us convert `RawVec::ptr` -> `Vec::as_ptr` -> `Vec::as_slice` -> `String::as_str`.

I had to move the `Deref` implementations into `as_{str,slice}` because `Deref` isn't `#[const_trait]`, but I would expect this change to be invisible up to inlining. I moved the `DerefMut` implementations as well for uniformity.

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