Update library functions with stability attributes

This may not be strictly minimal, but all unstable functions also need a
`rustc_const_unstable` attribute.

library/alloc/src/raw_vec.rs

@@ -150,6 +150,7 @@ impl<T> RawVec<T, Global> {
 impl<T, A: AllocRef> RawVec<T, A> {
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
+    #[allow_internal_unstable(const_fn)]
     pub const fn new_in(alloc: A) -> Self {
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr: Unique::dangling(), cap: 0, alloc }