Replace most uses of pointer::offset with add and sub

compiler/rustc_arena/src/lib.rs

@@ -217,7 +217,7 @@ impl<T> TypedArena<T> {
             } else {
                 let ptr = self.ptr.get();
                 // Advance the pointer.
-                self.ptr.set(self.ptr.get().offset(1));
+                self.ptr.set(self.ptr.get().add(1));
                 // Write into uninitialized memory.
                 ptr::write(ptr, object);
                 &mut *ptr