fixes rust-lang#62691

… a pattern

Added tests and stderr output

They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.

This also adds docs in the unstable book for the current state of auto traits.

permit negative impls for non-auto traits

This is a prototype impl that extends `impl !Trait` beyond auto traits. It is not integrated with coherence or anything else, and hence only serves to prevent downstream impls (but not to allow downstream crates to rely on the absence of such impls for coherence purposes).

Fixes rust-lang#66544

TODO:

- [x] need a test that you can't rely on negative impls for coherence purposes
- [x] test that negative impls cannot specialize positive ones
- [x] test that positive impls cannot specialize negative ones
- [x] extend negative impl to `Clone` in order to fully fix rust-lang#66544
- [x] and maybe make `CoerceUnsized` unsafe? -- that problem is now split out into rust-lang#68015
- [x] introduce feature flag and prepare a write-up
- [x] improve diagnostics?

Miri nits: comment and var name improvement

r? @eddyb

Fix for rust-lang#62691: use the largest niche across all fields

fixes rust-lang#62691

(The second commit is a small optimization but it makes the code less pretty and i don't know if it is worth it.)

parser: recover on `...` as a pattern, suggesting `..`

Fixes rust-lang#70388

My first PR to rust. So please let me know if I'm doing something wrong.

simplify match stmt

We actually have a surprising amount of
```rust
match expr {
    $($p:pat)|+ => true,
    _ => false,
}
```
While I would prefer this to be replaced with `matches!`, most cases are
fairly readable anyway so we can just let them be for now.