Closes rust-lang#84434. Closes rust-lang#82329.

Move all `sys::ext` modules to `os`

This PR moves all `sys::ext` modules to `os`, centralizing the location of all `os` code and simplifying the dependencies between `os` and `sys`.

Because this also removes all uses `cfg_if!` on publicly exported items, where after rust-lang#81969 there were still a few left, this should properly work around rust-lang/rust-analyzer#6038.

`@rustbot` label: +T-libs-impl

move core::hint::black_box under its own feature gate

The `black_box` function had its own RFC and is tracked separately from the `test` feature at rust-lang#64102. Let's reflect this in the feature gate.

To avoid breaking all the benchmarks, libtest's `test::black_box` is a wrapping definition, not a reexport -- this means it is still under the `test` feature gate.

…es, r=varkor

various const parameter defaults improvements

Actually resolve names in const parameter defaults, fixing `struct Foo<const N: usize = { usize::MAX }>`.

---
Split generic parameter ban rib for types and consts, allowing
```rust
#![feature(const_generics_defaults)]
struct Q;
struct Foo<T = Q, const Q: usize = 3>(T);
```

---
Remove the type/const ordering restriction if `const_generics_defaults` is active, even if `const_generics` is not. allowing us to stabilize and test const param defaults separately.

---
Check well formedness of const parameter defaults, eagerly emitting an error for `struct Foo<const N: usize = { 0 - 1 }>`

---
Do not forbid const parameters in param defaults, allowing `struct Foo<const N: usize, T = [u8; N]>(T)` and `struct Foo<const N: usize, const M: usize = N>`. Note that this should not change anything which is stabilized, as on stable, type parameters must be in front of const parameters, which means that type parameter defaults are only allowed if no const parameters exist.

We still forbid generic parameters inside of const param types.

r? ``@varkor`` ``@petrochenkov``

update Miri

Cc ``@rust-lang/miri`` r? ``@ghost``

…nt, r=petrochenkov

Handle pretty printing of `else if let` clauses without ICEing

When pretty printing the HIR of `if ... {} else if let ... {}` clauses, this displays it the `else if let` part as `match` it gets desugared to, the same way normal `if let` statements are currently displayed, instead of ICEing.

```rust
pub fn main() {
    if true {
        // 1
    } else if let a = 1 {
        // 2
    } else {
        // 3
    }
}
```

now gets desugared (via `rustc -Zunpretty=hir,typed src/x.rs`) to:

```rust
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
pub fn main() ({
                   (if (true as bool)
                       ({
                            // 1
                        } as
                           ()) else {match (1 as i32) {
                                         a => {
                                             // 2
                                         }
                                         _ => {
                                             // 3
                                         }
                                     }} as ())
                    } as ())
```

For comparison, this code gets HIR prettyprinted the same way before and after this change:

```rust
pub fn main() {
    if let a = 1 {
        // 2
    } else {
        // 3
    }
}
```
turns into
```rust
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
pub fn main() ({
                   (match (1 as i32) {
                        a => {
                            // 2
                        }
                        _ => {
                            // 3
                        }
                    } as ())
               } as ())
```

This closes rust-lang#82329. It closes rust-lang#84434 as well, due to having the same root cause.

Tweak trait not `use`d suggestion

Fix rust-lang#84272. Follow up to rust-lang#83667.

Add suggestion to "use break" when attempting to implicit-break a loop

Fixes rust-lang#84114