Merge branch 'feature/simplify-foreach' into 'master'

Short circuit Hybrid::forEach for integer_sequence

This reduced the template instanciation depth drastically.
E.g., for range=`integer_sequence<21>` is changed from 319 to 84.

This is especially important for clang, because this has a default max-depth of 256 in contrast to gcc (900).

See merge request !162

dune/common/hybridutilities.hh

@@ -262,6 +262,13 @@ namespace Impl {
     evaluateFoldExpression<int>({(f(Hybrid::elementAt(range, std::integral_constant<Index,i>())), 0)...});
   }
 
+  template<class F, class Index, Index... i>
+  constexpr void forEach(std::integer_sequence<Index, i...> range, F&& f, PriorityTag<2>)
+  {
+    evaluateFoldExpression<int>({(f(std::integral_constant<Index,i>()), 0)...});
+  }
+
+
   template<class Range, class F,
     std::enable_if_t<IsIntegralConstant<decltype(Hybrid::size(std::declval<Range>()))>::value, int> = 0>
   constexpr void forEach(Range&& range, F&& f, PriorityTag<1>)