Urgent bugfix: correctly sort data prep when using nested runners

``` r
munge(data, list(stagerunner, mungepiece, mungepieces))
```

will be OK but

``` r
munge(data, list(mungepiece1, stagerunner, mungepiece2))
```

will give the wrong result and attach `list(mungepieces_from_stagerunner, mungepiece1, mungepiece2)`
