- Remove unnecessary `should_loop` variable
- Report errors for trait implementations
These should give resolution errors because they are visible outside the
current scope. Without these errors, rustdoc will give ICEs:
```
thread 'rustc' panicked at 'attempted .def_id() on invalid res: Err', /home/joshua/src/rust/src/libstd/macros.rs:16:9
15: rustc_hir::def::Res<Id>::def_id
at /home/joshua/src/rust/src/librustc_hir/def.rs:382
16: rustdoc::clean::utils::register_res
at src/librustdoc/clean/utils.rs:627
17: rustdoc::clean::utils::resolve_type
at src/librustdoc/clean/utils.rs:587
```
- Add much more extensive tests
+ fn -> impl -> fn
+ fn -> impl -> fn -> macro
+ errors in function parameters
+ errors in trait bounds
+ errors in the type implementing the trait
+ unknown bounds for the type
+ unknown types in function bodies
+ errors generated by macros
- Use explicit state instead of trying to reconstruct it from random info
- Use an enum instead of a boolean
- Add example of ignored error
