Adding LexDFS and LexBFS

[ekanshjha]

It'll be awesome to see the implementation for lexicographically DFS and BFS.

[rafalk342]

It's actually quite simple to implement LexDFS in O(min(n^2, n+mloglog(n))).
It can also be implemented in linear time for cocomparability graphs as described here:
https://arxiv.org/pdf/1404.5996.pdf

[Dhairysheel-Deshmukh]

I would like to implement lex DFS and Lex BFS