Add directed graph data structure

[the-c0d3br34k3r]

• [ ] Description Add directed graph data structure.
  
• [ ] Use cases
  1. Save the world.
  2. Stop the imposter.
  3. ....
     
• [ ] Proposed design A clear and concise description of the design planned for this feature/enhancement. Attach design diagrams if applicable.
• [ ] Alternative design approaches A list of other approaches that could've been taken to design this feature/enhancement, but hasn't been because of a specified reason.
• [ ] Workaround If any workaround exists to achieve the desired goal without this feature/enhancement.
• [ ] Additional information Add any other context about the problem here.

[the-c0d3br34k3r]

Won't add as it is already present in Guava. Graphs - https://github.com/google/guava/wiki/GraphsExplained