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

src/libstd/sys_common/process.rs

@@ -11,16 +11,12 @@ use crate::sys::process::EnvKey;
 pub struct CommandEnv {
     clear: bool,
     saw_path: bool,
-    vars: BTreeMap<EnvKey, Option<OsString>>
+    vars: BTreeMap<EnvKey, Option<OsString>>,
 }
 
 impl Default for CommandEnv {
     fn default() -> Self {
-        CommandEnv {
-            clear: false,
-            saw_path: false,
-            vars: Default::default()
-        }
+        CommandEnv { clear: false, saw_path: false, vars: Default::default() }
     }
 }
 
@@ -64,11 +60,7 @@ impl CommandEnv {
     }
 
     pub fn capture_if_changed(&self) -> Option<BTreeMap<EnvKey, OsString>> {
-        if self.is_unchanged() {
-            None
-        } else {
-            Some(self.capture())
-        }
+        if self.is_unchanged() { None } else { Some(self.capture()) }
     }
 
     // The following functions build up changes