performance improvements for rbind.fill

This pull request will contain a performance test for `rbind.fill` and a fix that will change the run time to linear (instead of now quadratic).

The test tests the run time of `rbind.fill` for different list sizes and fits several linear models to check whether the dependence is O(n), O(n log n) or O(n^2). It runs in just a few seconds. These are the test results on my machine:

```
   .id .n user.self sys.self elapsed user.child sys.child   N     NlogN     N2
1   25  1     0.008    0.000   0.008          0         0  25   80.4719    625
2   25  2     0.008    0.000   0.007          0         0  25   80.4719    625
3   50  1     0.016    0.000   0.020          0         0  50  195.6012   2500
4   50  2     0.016    0.000   0.021          0         0  50  195.6012   2500
5  100  1     0.028    0.028   0.056          0         0 100  460.5170  10000
6  100  2     0.040    0.012   0.057          0         0 100  460.5170  10000
7  200  1     0.152    0.004   0.232          0         0 200 1059.6635  40000
8  200  2     0.152    0.004   0.271          0         0 200 1059.6635  40000
9  400  1     0.604    0.020   0.879          0         0 400 2396.5858 160000
10 400  2     0.540    0.020   0.774          0         0 400 2396.5858 160000
        .id r.squared
1    linear 0.9594638
2 loglinear 0.9748034
3   squared 0.9907632
```

The second table shows model fits for the different run time assumptions. Currently the run time clearly is O(N^2) for N = length of list.