NFC: Introduce skeletal ds/ptrwrap.h

This lets us go ahead and land several preparatory commits without waiting for
the real AuthPtr<> types to show up.

src/ds/ptrwrap.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <atomic>
+
+namespace snmalloc
+{
+  /**
+   * To assist in providing a uniform interface regardless of pointer wrapper,
+   * we also export intrinsic pointer and atomic pointer aliases, as the postfix
+   * type constructor '*' does not work well as a template parameter and we
+   * don't have inline type-level functions.
+   */
+  template<typename T>
+  using Pointer = T*;
+
+  template<typename T>
+  using AtomicPointer = std::atomic<T*>;
+} // namespace snmalloc