Format libstd with rustfmt
(Same strategy as #66691.)
This commit applies rustfmt with rust-lang/rust's default settings to files in src/libstd *that are not involved in any currently open PR* to minimize merge conflicts, and are not part of libstd/os (#66818) or libstd/sys (#66819). 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:
$ find src/libstd -name '*.rs' \
| xargs rustfmt --edition=2018 --unstable-features --skip-children
$ rg libstd outstanding_files | xargs git checkout --
Repeating this process several months apart should get us coverage of most of the rest of libstd.
To confirm no funny business:
$ git checkout $THIS_COMMIT^
$ git show --pretty= --name-only $THIS_COMMIT \
| xargs rustfmt --edition=2018 --unstable-features --skip-children
$ git diff $THIS_COMMIT # there should be no difference