Add nqueens benchmark (new kraken impl, with koka and cpp moved over from koka_bench)

koka_bench/cpp/CMakeLists.txt

@@ -2,7 +2,7 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
 set(CMAKE_CXX_EXTENSIONS NO)
 
-foreach (source IN ITEMS rbtree.cpp)
+foreach (source IN ITEMS rbtree.cpp nqueens.cpp)
   get_filename_component(name "${source}" NAME_WE)
   set(name "cpp-${name}")