Merge performance

Before

```
Benchmark                                       (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.flatMapInt                        1  thrpt         5   817098.263    26244.709    ops/s
r.u.PerfTransforms.flatMapInt                     1024  thrpt         5      933.212       13.307    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake        1  thrpt         5   691744.762    31131.700    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake     1024  thrpt         5       70.242        3.444    ops/s
```

After

```
Benchmark                                       (size)   Mode   Samples         Mean   Mean error    Units
r.u.PerfTransforms.flatMapInt                        1  thrpt         5   930876.214    37411.420    ops/s
r.u.PerfTransforms.flatMapInt                     1024  thrpt         5     1099.267       61.843    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake        1  thrpt         5   765968.829    51976.172    ops/s
r.u.PerfTransforms.flatMapNestedMapFilterTake     1024  thrpt         5       80.220        2.449    ops/s
```

Still longs way off from v0.19