Move MiniSet to data_structures

remove the need for T to be copy from MiniSet as was done for MiniMap

compiler/rustc_data_structures/src/lib.rs

@@ -102,6 +102,7 @@ pub mod work_queue;
 pub use atomic_ref::AtomicRef;
 pub mod frozen;
 pub mod mini_map;
+pub mod mini_set;
 pub mod tagged_ptr;
 pub mod temp_dir;
 pub mod unhash;