Format libcore with rustfmt

This commit 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:
https://gist.github.com/dtolnay/aa9c34993dc051a4f344d1b10e4487e8

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.

src/libcore/ops/function.rs

@@ -57,13 +57,16 @@
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
-    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
-    message="expected a `{Fn}<{Args}>` closure, found `{Self}`",
-    label="expected an `Fn<{Args}>` closure, found `{Self}`",
+    on(
+        Args = "()",
+        note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"
+    ),
+    message = "expected a `{Fn}<{Args}>` closure, found `{Self}`",
+    label = "expected an `Fn<{Args}>` closure, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-pub trait Fn<Args> : FnMut<Args> {
+pub trait Fn<Args>: FnMut<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
@@ -136,13 +139,16 @@ pub trait Fn<Args> : FnMut<Args> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
-    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
-    message="expected a `{FnMut}<{Args}>` closure, found `{Self}`",
-    label="expected an `FnMut<{Args}>` closure, found `{Self}`",
+    on(
+        Args = "()",
+        note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"
+    ),
+    message = "expected a `{FnMut}<{Args}>` closure, found `{Self}`",
+    label = "expected an `FnMut<{Args}>` closure, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-pub trait FnMut<Args> : FnOnce<Args> {
+pub trait FnMut<Args>: FnOnce<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
@@ -207,9 +213,12 @@ pub trait FnMut<Args> : FnOnce<Args> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
-    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
-    message="expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
-    label="expected an `FnOnce<{Args}>` closure, found `{Self}`",
+    on(
+        Args = "()",
+        note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"
+    ),
+    message = "expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
+    label = "expected an `FnOnce<{Args}>` closure, found `{Self}`"
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
@@ -225,8 +234,9 @@ pub trait FnOnce<Args> {
 
 mod impls {
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A,F:?Sized> Fn<A> for &F
-        where F : Fn<A>
+    impl<A, F: ?Sized> Fn<A> for &F
+    where
+        F: Fn<A>,
     {
         extern "rust-call" fn call(&self, args: A) -> F::Output {
             (**self).call(args)
@@ -234,8 +244,9 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A,F:?Sized> FnMut<A> for &F
-        where F : Fn<A>
+    impl<A, F: ?Sized> FnMut<A> for &F
+    where
+        F: Fn<A>,
     {
         extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
             (**self).call(args)
@@ -243,8 +254,9 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A,F:?Sized> FnOnce<A> for &F
-        where F : Fn<A>
+    impl<A, F: ?Sized> FnOnce<A> for &F
+    where
+        F: Fn<A>,
     {
         type Output = F::Output;
 
@@ -254,8 +266,9 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A,F:?Sized> FnMut<A> for &mut F
-        where F : FnMut<A>
+    impl<A, F: ?Sized> FnMut<A> for &mut F
+    where
+        F: FnMut<A>,
     {
         extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
             (*self).call_mut(args)
@@ -263,8 +276,9 @@ mod impls {
     }
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A,F:?Sized> FnOnce<A> for &mut F
-        where F : FnMut<A>
+    impl<A, F: ?Sized> FnOnce<A> for &mut F
+    where
+        F: FnMut<A>,
     {
         type Output = F::Output;
         extern "rust-call" fn call_once(self, args: A) -> F::Output {