Merge #3378

3378: vscode: redesign inlay hints to be capable of handling multiple editors for one source file r=Veetaha a=Veetaha

Fixes: #3008 (inlay corruption with multiple editors for one file).
Fixes: #3319 (unnecessary requests for inlay hints when switching unrelated source files or output/log/console windows)
Also, I don't know how, but the problem described in #3057 doesn't appear for me anymore (maybe it was some fix on the server-side, idk), the inlay hints are displaying right away. Though the last time I checked this it was caused by the server returning an empty array of hints and responding with a very big latency, I am not sure that this redesign actually fixed #3057....

We didn't handle the case when one rust source file is open in multiple editors in vscode (e.g. by manually adding another editor for the same file or by opening an inline git diff view or just any kind of preview within the same file).

The git diff preview is actually quite special because it causes memory leaks in vscode (https://github.com/microsoft/vscode/issues/91782). It is not removed from `visibleEditors` once it is closed. However, this bug doesn't affect the inlay hints anymore, because we don't issue a request and set inlay hints for each editor in isolation. Editors are grouped by their respective files and we issue requests only for files and then update all duplicate editors using the results (so we just update the decorations for already closed git diff preview read-only editors).

Also, note on a hack I had to use. `vscode.TextEdtior` identity is not actually public, its `id` field is not exposed to us. I created a dedicated upstream issue for this (https://github.com/microsoft/vscode/issues/91788).

Regarding #3319: the newly designed hints client doesn't issue requests for type hints when switching the visible editors if it has them already cached (though it does rerender things anyway, but this could be optimized in future if so wanted).

<details>
<summary>Before</summary>

![bug_demo](https://user-images.githubusercontent.com/36276403/75613171-3cd0d480-5b33-11ea-9066-954fb2fb18a5.gif)


</details>

<details>
<summary> After </summary>

![multi-cursor-replace](https://user-images.githubusercontent.com/36276403/75612710-d5b12100-5b2e-11ea-99ba-214b4219e6d3.gif)

</details>

Co-authored-by: Veetaha <[email protected]>

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