Use parallel-hashmap instead of sparsepp

[boschmitt]

While trying to fix some warnings on my side, I found that the author of sparsepp recommends the use of his new header-only library parallel-hashmap: https://github.com/greg7mdp/parallel-hashmap

I suppose the change should be straightforward.

I'm not sure how much of an impact it would make on mockturtle performance, but the author says that parallel-hashmap is significantly faster.