clean up %ToString calls with a to_string helper, and avoid copying exprs in make_array_set
clean up %ToString calls with a to_string helper, and avoid copying exprs in make_array_set

logchi/LambdaS5,lijunsong/LambdaS5,brownplt/LambdaS5,brownplt/LambdaS5,brownplt/LambdaS5,logchi/LambdaS5,logchi/LambdaS5,lijunsong/LambdaS5,lijunsong/LambdaS5