Fix IterExt for arrays.

It's implemented for `std::array::IntoIter<T, N>` but that impl is never
used. This commit changes it to an impl for `[T; N]` which does get used
in various `alloc_from_iter([a, b])` call sites.

(Note: I used logging to confirm that the old impl was never used and
that the new one is used.)

Author: nnethercote

compiler/rustc_arena/src/lib.rs

@@ -133,7 +133,7 @@ where
     }
 }
 
-impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> {
+impl<T, const N: usize> IterExt<T> for [T; N] {
     #[inline]
     fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
         let len = self.len();