Add default ctor ability to gc policy's edit

Author: jeaye

immer/transience/gc_transience_policy.hpp

@@ -39,12 +39,8 @@ struct gc_transience_policy
 
             struct edit
             {
-                void* v;
-                edit(void* v_)
-                    : v{v_}
-                {
-                }
-                edit() = delete;
+                void* v{};
+
                 bool operator==(edit x) const { return v == x.v; }
                 bool operator!=(edit x) const { return v != x.v; }
             };