Auto merge of #66691 - dtolnay:fmt0, r=sfackler

Format libcore with rustfmt

I am interested in whether we can begin cautious incremental progress on #66688 and assess along the way whether we can keep the disruption sufficiently small.

This PR applies rustfmt with default settings to files in src/libcore *that are not involved in any currently open PR* to minimize merge conflicts. The list of files involved in open PRs was determined by querying GitHub's GraphQL API  [with this script](https://gist.github.com/dtolnay/aa9c34993dc051a4f344d1b10e4487e8).

With the list of files from the script in `outstanding_files`, the relevant commands were:

```console
$ find src/libcore -name '*.rs' | xargs rustfmt --edition=2018
$ rg libcore outstanding_files | xargs git checkout --
```

Repeating this process several months apart should get us coverage of most of the rest of libcore.

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