This enables us to set more generic labels shared between targets. For
example `target_family="wasm"` across all targets that are conceptually
"wasm".

See rust-lang/reference#1006

… r=petrochenkov

Allow setting `target_family` to multiple values, and implement `target_family="wasm"`

As per the conclusion in [this thread](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Are.20we.20comfortable.20with.20adding.20an.20insta-stable.20cfg.28wasm.29.3F/near/233158441), this implements an ability to specify any number of `target_family` values, allowing for more flexible generic groups, or "families", to be created than just the OS-based unix/windows dichotomy.

cc rust-lang/reference#1006

Add ErrorKind::OutOfMemory

Ability to express `ENOMEM` as an `io::Error`.

I've used `OutOfMemory` as opposed to `NotEnoughMem` or `AllocationFailed`, because "OOM" is used in Rust already.

See also rust-lang#84612

Add help message to suggest const for unused type param

r? `@lcnr`

…=jyn514

RustDoc: Fix bounds linking trait.Foo instead of traitalias.Foo

Fixes rust-lang#84782

The code was assuming `Trait` when adding bounds to the cache, so add a check on the DefId to see what its kind really is.

r? `@jyn514`

Before:
![image](https://user-images.githubusercontent.com/18526288/116775611-6a751e80-aa53-11eb-84d0-ed6b7782be3c.png)

After:
![image](https://user-images.githubusercontent.com/18526288/116802227-d19cdc80-ab00-11eb-8133-7b34dd750da2.png)

suggestion for unit enum variant when matched with a patern

resolve rust-lang#84700

add suggestion for code like
```rust
enum FarmAnimal {
    Worm,
    Cow,
    Bull,
    Chicken { num_eggs: usize },
    Dog (String),
}

fn what_does_the_animal_say(animal: &FarmAnimal) {

    let noise = match animal {
        FarmAnimal::Cow(_) => "moo".to_string(),
        _ => todo!()
    };

    println!("{:?} says: {:?}", animal, noise);
}
```

```
error[E0532]: expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
  --> $DIR/issue-84700.rs:15:9
   |
LL |     Cow,
   |     --- `FarmAnimal::Cow` defined here
...
LL |         FarmAnimal::Cow(_) => "moo".to_string(),
   |         ^^^^^^^^^^^^^^^^^^ help: use this syntax instead: `FarmAnimal::Cow`
   ```

…l-traits, r=jyn514

Do not print visibility in external traits

This PR fixes the bug that caused traits, which were re-exported, having visibility modifiers in front of methods, which is invalid.

It would be nice to add a test for this, but I don't even know if tests with multiple crates are possible.

Resolves rust-lang#81274