1515: Trait environment r=matklad a=flodiebold
This adds the environment, i.e. the set of `where` clauses in scope, when solving trait goals. That means that e.g. in
```rust
fn foo<T: SomeTrait>(t: T) {}
```
, we are able to complete methods of `SomeTrait` on the `t`. This affects the trait APIs quite a bit (since every method that needs to be able to solve for some trait needs to get this environment somehow), so I thought I'd do it rather sooner than later ;)
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
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