Rollup merge of #131120 - tgross35:stabilize-const_option, r=RalfJung

Stabilize `const_option`

This makes the following API stable in const contexts:

```rust
impl<T> Option<T> {
    pub const fn as_mut(&mut self) -> Option<&mut T>;
    pub const fn expect(self, msg: &str) -> T;
    pub const fn unwrap(self) -> T;
    pub const unsafe fn unwrap_unchecked(self) -> T;
    pub const fn take(&mut self) -> Option<T>;
    pub const fn replace(&mut self, value: T) -> Option<T>;
}

impl<T> Option<&T> {
    pub const fn copied(self) -> Option<T>
    where T: Copy;
}

impl<T> Option<&mut T> {
    pub const fn copied(self) -> Option<T>
    where T: Copy;
}

impl<T, E> Option<Result<T, E>> {
    pub const fn transpose(self) -> Result<Option<T>, E>
}

impl<T> Option<Option<T>> {
    pub const fn flatten(self) -> Option<T>;
}
```

The following functions make use of the unstable `const_precise_live_drops` feature:

- `expect`
- `unwrap`
- `unwrap_unchecked`
- `transpose`
- `flatten`

Fixes: <rust-lang/rust#67441>

src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(rustc_private)]
 #![feature(cell_update)]
-#![feature(const_option)]
 #![feature(float_gamma)]
 #![feature(map_try_insert)]
 #![feature(never_type)]