These show gains from the recent optimization commits:

```
                        arg_splat
            miniruby:   7346039.9 i/s
     miniruby-before:   4692240.8 i/s - 1.57x  slower

                  arg_splat_block
            miniruby:   6539749.6 i/s
     miniruby-before:   4358063.6 i/s - 1.50x  slower

                   splat_kw_splat
            miniruby:   5433641.5 i/s
     miniruby-before:   3851048.6 i/s - 1.41x  slower

             splat_kw_splat_block
            miniruby:   4916137.1 i/s
     miniruby-before:   3477090.1 i/s - 1.41x  slower

                   splat_kw_block
            miniruby:   2912829.5 i/s
     miniruby-before:   2465611.7 i/s - 1.18x  slower

                   arg_splat_post
            miniruby:   2195208.2 i/s
     miniruby-before:   1860204.3 i/s - 1.18x  slower
```

zsuper only speeds up in the post argument case, because
it was already set to use splatarray false in cases where
there were no post arguments.