Add a parallel Haskell version from my levgen-benchmarks repo.

Atom D525 100 levels,
N,time (seconds)
1,3.68
2,1.8
3,1.43
4,1.23
,
i7-3610QM 100 levels,
1,0.475
2,0.311
3,0.232
4,0.171
5,0.163
6,0.153
7,0.149
8,0.148
,
i7-3610QM 1000 levels,
1,4.43
2,2.36
3,1.69
4,1.47
5,1.33
6,1.18
7,1.12
8,1.13