Merge #3966 #3968

3966: Add support for bounds on associated types in trait definitions r=matklad a=flodiebold

E.g.
```rust
trait Trait {
    type Item: SomeOtherTrait;
}
```
Note that these don't simply desugar to where clauses; as I understand it, where clauses have to be proved by the *user* of the trait, but these bounds are proved by the *implementor*. (Also, where clauses on associated types are unstable.)

(Another one from my recursive solver branch...)

3968: Remove format from syntax_bridge hot path r=matklad a=edwin0cheng

Although only around 1% speed up by running:

```
Measure-Command {start-process .\target\release\rust-analyzer "analysis-stats -q ." -NoNewWindow -wait}
```

Co-authored-by: Florian Diebold <flodiebold@gmail.com>
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>