Add const versions of markptr<T>::ptr and mark

Otherwise a const markptr isn't all that useful.

Add const versions of markptr<T>::ptr and mark
d17c6eab · David Benjamin · 9de4be61 · d17c6eab
--- a/include/markptr.hh
+++ b/include/markptr.hh
@@ -25,10 +25,16 @@ class markptr {
  markptr_ptr<T>& ptr() {
    return *(markptr_ptr<T>*) this;
  }
+  const markptr_ptr<T>& ptr() const {
+    return *(const markptr_ptr<T>*) this;
+  }
  markptr_mark<T>& mark() {
    return *(markptr_mark<T>*) this;
  }
+  const markptr_mark<T>& mark() const {
+    return *(const markptr_mark<T>*) this;
+  }
  // Convenience operator to avoid having to write out xx.ptr()->...
  T* operator->() { return ptr(); }