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, 3 insertions, 4 deletions

diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 2df79ee97fb..11850a1b5fc 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -313,9 +313,8 @@ macro_rules! dbg {
 
 #[cfg(test)]
 macro_rules! assert_approx_eq {
-    ($a:expr, $b:expr) => ({
+    ($a:expr, $b:expr) => {{
         let (a, b) = (&$a, &$b);
-        assert!((*a - *b).abs() < 1.0e-6,
-                "{} is not approximately equal to {}", *a, *b);
-    })
+        assert!((*a - *b).abs() < 1.0e-6, "{} is not approximately equal to {}", *a, *b);
+    }};
 }