Merge #6130 #6135

6130: Items case quick fix (snake_case / UPPER_SNAKE_CASE / CamelCase) r=matklad a=popzxc

Resolves #4598.

After a third try, it finally works. Boy, it appeared tougher than it seemed.

Initially I thought like "Ha, `rustc` already tells us where idents are named incorrectly. It shouldn't be that hard, should it?".

Well, the problems with the information provided by `rustc` appeared shortly:

- `rustc` warnings are `flycheck` warnings, which are slightly aside from our diagnostics with fixes.
  When we map flycheck diagnostic to LSP, we can convert it into a fix, but only if it's marked as `Applicability::MachineApplicable`.
  Name case fix is marked `Applicability::MaybeIncorrect`, and for a reason: it only suggest to rename symbol under cursor, without tracking any references.
- Warning spawned by `rustc` are identified by string labels rather than enum. It means that if one day the diagnostic will be renamed in `rustc`, `rust-analyzer` code will still compile, but won't find the required diagnostic by name anymore. If by chance this will happen when some unlucky guy will decide to create their first pull request, they'll be confused by suddenly failing tests  (likely) not related to their changes.
- Even if we'll try to build fixes atop of `rustc` warnings, we'll have to do it in the `rust_analyzer::diagnostics::to_proto` module, which is far less
  convenient for that matter than `ide` crate.

That's why I decided that it's worth a separate `rust-analyzer` diagnostic, which will implement `DiagnosticWithFix` trait.

After that, I discovered that currently `hir_ty::diagnostics` only check `DefWithBody` types, like function bodies. I had to add support for diagnostics
which look at any `ModuleDef`.

And of course, since I'd added a lot of new functionality, it required extensive testing.

That explains why the diff is so big for a (looking) relatively small feature.

I hope that this PR doesn't only add a small feature, but also creates a base for building another features.

## Example:

![case_quick_fix](https://user-images.githubusercontent.com/12111581/95008475-e07ee780-0622-11eb-9978-62a9ea0e7782.gif)

P.S. My eyes were bleeding when I had to write the code for the example...


6135: when generating new function, focus on return type instead of body r=matklad a=bnjjj

I made a little change when we use the assist to generate a new function, instead of focusing on the function body, it will focus on return type

Co-authored-by: Igor Aleksanov <[email protected]>
Co-authored-by: Benjamin Coenen <[email protected]>