Rollup merge of #47833 - Aaron1011:final_auto_trait, r=GuillaumeGomez

Generate documentation for auto-trait impls

A new section is added to both both struct and trait doc pages.

On struct/enum pages, a new 'Auto Trait Implementations' section displays any synthetic implementations for auto traits. Currently, this is only done for Send and Sync.

![Auto trait implementations for Cloned](https://i.imgur.com/XtTV6IJ.png)

On trait pages, a new 'Auto Implementors' section displays all types which automatically implement the trait. Effectively, this is a list of all public types in the standard library.

![Auto trait implementors for Send](https://i.imgur.com/3GRBpTy.png)

Synthesized impls for a particular auto trait ('synthetic impls') take generic bounds into account. For example, a type
```rust
struct Foo<T>(T)
```
 will have 'impl<T> Send for Foo<T> where T: Send' generated for it.

Manual implementations of auto traits are also taken into account. If we have
the following types:

```rust
struct Foo<T>(T)
struct Wrapper<T>(Foo<T>)
unsafe impl<T> Send for Wrapper<T>' // pretend that Wrapper<T> makes this sound somehow
```

Then Wrapper will have the following impl generated:
```rust
impl<T> Send for Wrapper<T>
```
reflecting the fact that 'T: Send' need not hold for 'Wrapper<T>: Send' to hold

Lifetimes, HRTBS, and projections (e.g. '<T as Iterator>::Item') are taken into account by synthetic impls:

![A ridiculous demonstration type](https://i.imgur.com/TkZMWuN.png)

However, if a type can *never* implement a particular auto trait (e.g. `struct MyStruct<T>(*const T)`), then a negative impl will be generated (in this case, `impl<T> !Send for MyStruct<T>`)

All of this means that a user should be able to copy-paste a syntheticimpl into their code, without any observable changes in behavior (assuming the rest of the program remains unchanged).