Add algorithms on basic trees

[acorrenson]

I implemented n-ary trees and binary trees in OCaml

• With BFS and DFS{PRE, POST, INF} traversal
• With clean .dot format pretty printers

[imartayan]

Using a queue would be more efficient than a list for the bfs. We can use the Queue module from the standard library, or we can also implement our own queue module.

[acorrenson]

I agree, but my point wasn't efficiency but code readability. I'm going to implement a stronger append and a stonger map (using continuations). You can add another (more efficient) version of the BFS if u want ^^

[Moxinilian]

Personally I would be in favor of using the Queue module. Not only is it more explicit and readable, but it moves the responsibility of performance to somebody else.

[acorrenson]

Using the queue module is a bit cheating since this is an educational project ^^. I'm going to implement functionnal queues (based on the book of Conchon and Filliâtre)