Draft: Add experimental standard scope guards

This adds an implementation for scoped guards proposed for the standard library: https://en.cppreference.com/w/cpp/experimental/lib_extensions_3. We use guards in some places but they are very basic. It would be nice to have a proper solution, so this is to a proper implementation of the generic guard for the experimental std::scope_[exit|fail|success].

dune/common/std/test/CMakeLists.txt

@@ -21,6 +21,10 @@ dune_add_test(SOURCES mdarraytest.cc
               LABELS quick
               LINK_LIBRARIES Dune::Common)
 
+dune_add_test(SOURCES scopetest.cc
+              LINK_LIBRARIES dunecommon
+              LABELS quick)
+
 dune_add_test(SOURCES spantest.cc
               LABELS quick
-              LINK_LIBRARIES Dune::Common)
\ No newline at end of file
+              LINK_LIBRARIES Dune::Common)