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`
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