Addition of nqueens benchmark to multicore-numerical

[ctk21]

This PR adds the (classical) nqueens benchmark to sandmark where the number of 'valid' configurations for n queens on a chessboard of size n is calculated.

We add both a single core and multicore version that uses tasks in a similar manner to how the Cilk nqueens benchmark.

[ctk21]

This PR currently has a problem that needs sorting out. If I run the multicore variant with Domainslib 0.2.2 then I'm getting a stack overflow:

for i in {4..14}; do ./nqueens_multicore.exe 4 $i ; done
2 solutions for board of size 4
10 solutions for board of size 5
4 solutions for board of size 6
40 solutions for board of size 7
92 solutions for board of size 8
352 solutions for board of size 9
724 solutions for board of size 10
2680 solutions for board of size 11
Fatal error: exception Stack_overflow
Fatal error: exception Stack_overflow
Fatal error: exception Stack_overflow

[Sudha247]

Running this benchmark on domainslib 0.3.0 does not produce a stack overflow anymore. I think switching to Multi_channel for storing async tasks eliminated the stack overflow. I suggest we merge this PR now as is, possibly after a rebase to master.

[ctk21]

Now rebased and with a run config file.
We need #253 to get things building for the parallel benchmarks.

[ctk21]

rebased now that #253 is merged. This one should now be good to merge.

[kayceesrk]

We need a config run config on navajo: https://github.com/ocaml-bench/sandmark/blob/master/multicore_parallel_navajo_run_config.json.

[Sudha247]

Closing this as it was merged in #329. Thanks for the benchmark @ctk21!