Rollup merge of rust-lang#116559 - Kritzefitz:btree-new-in-const, r=Amanieu

Mark `new_in` as `const` for BTree collections

Discussed in and closes rust-lang/wg-allocators#118

Author: GuillaumeGomez

alloc/src/collections/btree/map.rs

@@ -669,7 +669,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// map.insert(1, "a");
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
-    pub fn new_in(alloc: A) -> BTreeMap<K, V, A> {
+    pub const fn new_in(alloc: A) -> BTreeMap<K, V, A> {
         BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
     }
 }

alloc/src/collections/btree/set.rs

@@ -358,7 +358,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// let mut set: BTreeSet<i32> = BTreeSet::new_in(Global);
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
-    pub fn new_in(alloc: A) -> BTreeSet<T, A> {
+    pub const fn new_in(alloc: A) -> BTreeSet<T, A> {
         BTreeSet { map: BTreeMap::new_in(alloc) }
     }