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`

library/std/src/io/stdio.rs

@@ -301,6 +301,7 @@ pub struct StdinLock<'a> {
 ///     Ok(())
 /// }
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdin() -> Stdin {
     static INSTANCE: SyncOnceCell<Mutex<BufReader<StdinRaw>>> = SyncOnceCell::new();
@@ -674,6 +675,7 @@ static STDOUT: SyncOnceCell<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = Sy
 ///     Ok(())
 /// }
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdout() -> Stdout {
     Stdout {
@@ -954,6 +956,7 @@ pub struct StderrLock<'a> {
 ///     Ok(())
 /// }
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
     // Note that unlike `stdout()` we don't use `at_exit` here to register a