Format libstd with rustfmt

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. 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

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/backtrace.rs b/src/libstd/backtrace.rs
index 9f400713a86..5ba1c940251 100644
--- a/src/libstd/backtrace.rs
+++ b/src/libstd/backtrace.rs
@@ -95,10 +95,10 @@ use crate::env;
 use crate::fmt;
 use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 use crate::sync::Mutex;
-use crate::sys_common::backtrace::{output_filename, lock};
+use crate::sys_common::backtrace::{lock, output_filename};
 use crate::vec::Vec;
-use backtrace_rs as backtrace;
 use backtrace::BytesOrWideString;
+use backtrace_rs as backtrace;
 
 /// A captured OS thread stack backtrace.
 ///