Use LUT for group indexes instead of calculating on-the-fly

.gitignore

@@ -2,6 +2,7 @@
 *.o
 sudoku
 eval
+genlut
 
 # Test data
 sudoku.csv
@@ -15,3 +16,6 @@ compile_commands.json
 
 # Profiling results
 gmon.out
+
+# Generated code
+lut.c