Summary

This MR removes some c++ feature checks from cmake, since the features are already standard:

• DUNE_HAVE_CXX_BOOL_CONSTANT
• DUNE_HAVE_CXX_EXPERIMENTAL_BOOL_CONSTANT
• DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS
• DUNE_HAVE_CXX_APPLY
• DUNE_HAVE_CXX_EXPERIMENTAL_APPLY
• HAVE_IS_INDEXABLE_SUPPORT

The fallback implementations are removed and the inclusion into the Dune::Std namespace is marked deprecated. This does typically not work for using declarations. Thus, the functions are marked deprecated in Doxygen only.