Rollup merge of #131784 - Urgau:stabilize-midpoint, r=dtolnay

Stabilize unsigned and float variants of `num_midpoint` feature

This PR proposes that we stabilize the unsigned variants of the [`num_midpoint`](https://github.com/rust-lang/rust/issues/110840#issue-1684506201) feature as well as the floats variants, since they are not subject to any unresolved questions, which is equivalent to doing `(a + b) / 2` (and `(a + b) >> 1`) in a sufficiently large number.

The stabilized API surface would be:

```rust
/// Calculates the middle point of `self` and `rhs`.
///
/// `midpoint(a, b)` is `(a + b) / 2` as if it were performed in a sufficiently-large unsigned integral type.
/// This implies that the result is always rounded towards negative infinity and that no overflow will ever occur.

impl u{8,16,32,64,128,size} {
    pub const fn midpoint(self, rhs: Self) -> Self;
}

impl NonZeroU{8,16,32,64,size} {
    pub const fn midpoint(self, rhs: Self) -> Self;
}

impl f{32,64} {
    pub const fn midpoint(self, rhs: Self) -> Self;
}
```

The signed variants `u{8,16,32,64,128,size}` would remain gated, until a decision is made about the rounding mode, in other words that the [unresolved questions](https://github.com/rust-lang/rust/issues/110840#issue-1684506201) are resolved.

cc `@rust-lang/libs-api`
cc `@scottmcm`
r? libs-api

0 files changed, 0 insertions, 0 deletions