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>
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