Format libcore with rustfmt

[dtolnay]

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.

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

$ 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.