15: Add solution (part 1) (much faster)

 a) OCaml doesn't have a reasonable Priority Queue. Set is unusable.

 b) Turns out I don't need to keep «all» the elements in the unvisited set.
    Someone should strip me of my Master's Degree.

 c) Turns out ⟦v ∈ Unvisited⟧ can be also written as ⟦v ∉ Visited⟧ if we keep
    both sets updated. Someone should strip me of my high school graduation
    certificate as well.