Use arrayvec 0.7, drop smallvec 0.6

With the arrival of min const generics, many alt-vec libraries have
updated to use it in some way and arrayvec is no exception. Use the
latest with minor refactoring.

Also, rustc_workspace_hack is the only user of smallvec 0.6 in the
entire tree, so drop it.

compiler/rustc_index/src/bit_set.rs

@@ -375,7 +375,7 @@ const SPARSE_MAX: usize = 8;
 #[derive(Clone, Debug)]
 pub struct SparseBitSet<T> {
     domain_size: usize,
-    elems: ArrayVec<[T; SPARSE_MAX]>,
+    elems: ArrayVec<T, SPARSE_MAX>,
 }
 
 impl<T: Idx> SparseBitSet<T> {