Add support for C++20 concepts

• Some compilers already have support for concepts, hence, it would be a good idea to provide some basic compiler checks to enable it through DUNE.
• Additionally, since some compilers do not provide the concepts library (e.g. clang), some concepts are provided as a fallback implementation.

In particular, this would enable one of my two Grid concept implementations for those using C++20.

I tested this with clang-11. Pipelines for supported compilers seem to be passing.

Usage: dune-grid!377 (merged)

cmake/modules/CheckCXXFeatures.cmake

@@ -471,3 +471,9 @@ check_cxx_symbol_exists(
   "functional"
   DUNE_HAVE_CXX_STD_IDENTITY
   )
+
+check_cxx_source_compiles("
+  template<class T> concept C = false;
+  int main() {return C<int>;}"
+  DUNE_HAVE_CXX_CONCEPTS
+)
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