Add RustcVacantEntry::insert_entry for rust-lang/rust#64656

See rust-lang/rust#64656.

~~This was based on v0.5.0 as that's what rustc uses; I can rebase onto master, but I'm not sure whether rustc wants v0.6.0 or if v0.6.0 is rustc-ready.~~

For context, this ultimately provides an API with this shape:

```rust
impl Entry<'a, K, V> {
    fn insert(self, value: V) -> OccupiedEntry<'a, K, V> {…}
}
```

to be used when one wants to insert a value without consuming the Entry, e.g. because one wants to keep a reference to the key around. There's more at the original (defunct) PR: rust-lang/rust#60142.

Implement (HashMap) Entry::insert as per rust-lang#60142

Implementation of `Entry::insert` as per @SimonSapin's comment on rust-lang#60142. This requires a patch to hashbrown:

```diff
diff --git a/src/rustc_entry.rs b/src/rustc_entry.rs
index fefa5c3..7de8300 100644
--- a/src/rustc_entry.rs
+++ b/src/rustc_entry.rs
@@ -546,6 +546,32 @@ impl<'a, K, V> RustcVacantEntry<'a, K, V> {
         let bucket = self.table.insert_no_grow(self.hash, (self.key, value));
         unsafe { &mut bucket.as_mut().1 }
     }
+
+    /// Sets the value of the entry with the RustcVacantEntry's key,
+    /// and returns a RustcOccupiedEntry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use hashbrown::HashMap;
+    /// use hashbrown::hash_map::RustcEntry;
+    ///
+    /// let mut map: HashMap<&str, u32> = HashMap::new();
+    ///
+    /// if let RustcEntry::Vacant(v) = map.rustc_entry("poneyland") {
+    ///     let o = v.insert_and_return(37);
+    ///     assert_eq!(o.get(), &37);
+    /// }
+    /// ```
+     #[inline]
+    pub fn insert_and_return(self, value: V) -> RustcOccupiedEntry<'a, K, V> {
+        let bucket = self.table.insert_no_grow(self.hash, (self.key, value));
+        RustcOccupiedEntry {
+            key: None,
+            elem: bucket,
+            table: self.table
+        }
+    }
 }

 impl<K, V> IterMut<'_, K, V> {
```

This is also only an implementation for HashMap. I tried implementing for BTreeMap, but I don't really understand BTreeMap's internals and require more guidance on implementing the equivalent `VacantEntry::insert_and_return` such that it returns an `OccupiedEntry`. Notably, following the original PR's modifications I end up needing a `Handle<NodeRef<marker::Mut<'_>, _, _, marker::LeafOrInternal>, _>` while I only have a `Handle<NodeRef<marker::Mut<'_>, _, _, marker::Internal>, _>` and don't know how to proceed.

(To be clear, I'm not asking for guidance right now; I'd be happy getting only the HashMap implementation — the subject of this PR — reviewed and ready, and leave the BTreeMap implementation for a latter PR.)

Rollup of 6 pull requests

Successful merges:

 - #64656 (Implement (HashMap) Entry::insert as per #60142)
 - #64986 (Function pointers as const generic arguments)
 - #65037 (`#[track_caller]` feature gate (RFC 2091 1/N))
 - #65166 (Suggest to add `move` keyword for generator capture)
 - #65175 (add more info in debug traces for gcu merging)
 - #65220 (Update LLVM for Emscripten exception handling support)

Failed merges:

r? @ghost

Implement (HashMap) Entry::insert as per rust-lang#60142

Implementation of `Entry::insert` as per @SimonSapin's comment on rust-lang#60142. This requires a patch to hashbrown:

```diff
diff --git a/src/rustc_entry.rs b/src/rustc_entry.rs
index fefa5c3..7de8300 100644
--- a/src/rustc_entry.rs
+++ b/src/rustc_entry.rs
@@ -546,6 +546,32 @@ impl<'a, K, V> RustcVacantEntry<'a, K, V> {
         let bucket = self.table.insert_no_grow(self.hash, (self.key, value));
         unsafe { &mut bucket.as_mut().1 }
     }
+
+    /// Sets the value of the entry with the RustcVacantEntry's key,
+    /// and returns a RustcOccupiedEntry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use hashbrown::HashMap;
+    /// use hashbrown::hash_map::RustcEntry;
+    ///
+    /// let mut map: HashMap<&str, u32> = HashMap::new();
+    ///
+    /// if let RustcEntry::Vacant(v) = map.rustc_entry("poneyland") {
+    ///     let o = v.insert_and_return(37);
+    ///     assert_eq!(o.get(), &37);
+    /// }
+    /// ```
+     #[inline]
+    pub fn insert_and_return(self, value: V) -> RustcOccupiedEntry<'a, K, V> {
+        let bucket = self.table.insert_no_grow(self.hash, (self.key, value));
+        RustcOccupiedEntry {
+            key: None,
+            elem: bucket,
+            table: self.table
+        }
+    }
 }

 impl<K, V> IterMut<'_, K, V> {
```

This is also only an implementation for HashMap. I tried implementing for BTreeMap, but I don't really understand BTreeMap's internals and require more guidance on implementing the equivalent `VacantEntry::insert_and_return` such that it returns an `OccupiedEntry`. Notably, following the original PR's modifications I end up needing a `Handle<NodeRef<marker::Mut<'_>, _, _, marker::LeafOrInternal>, _>` while I only have a `Handle<NodeRef<marker::Mut<'_>, _, _, marker::Internal>, _>` and don't know how to proceed.

(To be clear, I'm not asking for guidance right now; I'd be happy getting only the HashMap implementation — the subject of this PR — reviewed and ready, and leave the BTreeMap implementation for a latter PR.)

Rollup of 4 pull requests

Successful merges:

 - #64656 (Implement (HashMap) Entry::insert as per #60142)
 - #65037 (`#[track_caller]` feature gate (RFC 2091 1/N))
 - #65166 (Suggest to add `move` keyword for generator capture)
 - #65175 (add more info in debug traces for gcu merging)

Failed merges:

r? @ghost