Added the `!queens` command

This command implements the n queens problem. As a safeguard, only up to n=14 is
calculated, otherwise the bot times out.
Updated and optimized the n queens problem.

I finally understood how foldM on lists work :happy: Since this provides an
efficient way of backtracking, no guard is needed for reasonable numbers
anymore.
Merge pull request #17 from keneanung/Queens

Adding nqueens component. Also updates a few dependency versions, allowing an easier build from scratch.