Rollup merge of #90430 - jkugelman:must-use-std-a-through-n, r=joshtriplett

Add #[must_use] to remaining std functions (A-N)

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is half of the remaining items from the `std` crate, from A-N.

I added these functions myself. Clippy predictably ignored the `mut` ones, but I don't know why the rest weren't flagged. Check them closely, please? Maybe I overlooked good reasons.

```rust
std::backtrace::Backtrace                                   const fn disabled() -> Backtrace;
std::backtrace::Backtrace<'a>                               fn frames(&'a self) -> &'a [BacktraceFrame];
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn key_mut(&mut self) -> &mut K;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_mut(&mut self) -> &mut V;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value(&mut self) -> (&K, &V);
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value_mut(&mut self) -> (&mut K, &mut V);
std::env                                                    fn var_os<K: AsRef<OsStr>>(key: K) -> Option<OsString>;
std::env                                                    fn split_paths<T: AsRef<OsStr> + ?Sized>(unparsed: &T) -> SplitPaths<'_>;
std::io::Error                                              fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)>;
```

Parent issue: #89692

r? `@joshtriplett`

src/test/ui/rust-2018/uniform-paths/redundant.rs

@@ -13,8 +13,8 @@ mod bar {
 }
 
 fn main() {
-    io::stdout();
-    self::std::io::stdout();
-    foo::my_std::io::stdout();
-    bar::std::io::stdout();
+    let _ = io::stdout();
+    let _ = self::std::io::stdout();
+    let _ = foo::my_std::io::stdout();
+    let _ = bar::std::io::stdout();
 }

src/test/ui/uniform-paths/basic-nested.rs

@@ -44,10 +44,10 @@ mod bar {
 
 fn main() {
     foo::Foo(());
-    foo::std_io::stdout();
+    let _ = foo::std_io::stdout();
     foo::local_io(());
-    io::stdout();
-    bar::io::stdout();
+    let _ = io::stdout();
+    let _ = bar::io::stdout();
     bar::std();
     bar::std!();
 
@@ -56,6 +56,6 @@ fn main() {
         // scope is allowed, when both resolve to the same definition.
         use std::io;
         use io::stdout;
-        stdout();
+        let _ = stdout();
     }
 }

src/test/ui/uniform-paths/basic.rs

@@ -20,7 +20,7 @@ use self::std::io as local_io;
 
 fn main() {
     Foo(());
-    std_io::stdout();
+    let _ = std_io::stdout();
     local_io(());
 
     {
@@ -28,6 +28,6 @@ fn main() {
         // scope is allowed, when both resolve to the same definition.
         use ::std::io as std_io;
         use std_io::stdout;
-        stdout();
+        let _ = stdout();
     }
 }

src/test/ui/uniform-paths/macros-nested.rs

@@ -46,8 +46,8 @@ mod bar {
 
 fn main() {
     foo::Foo(());
-    foo::std_io::stdout();
+    let _ = foo::std_io::stdout();
     foo::local_io(());
-    io::stdout();
-    bar::io::stdout();
+    let _ = io::stdout();
+    let _ = bar::io::stdout();
 }

src/test/ui/uniform-paths/macros.rs

@@ -31,6 +31,6 @@ m2!();
 
 fn main() {
     Foo(());
-    std_io::stdout();
+    let _ = std_io::stdout();
     local_io(());
 }