Make more use of haskell typeclasses

[jtanguy]

Instead of defining a Tree as a data, we could define it as a typeclass:

class Tree a where
    eval :: Tree -> a
    derive :: Tree -> Tree
    replace :: Tree -> a -> Tree
    clean :: Tree -> Tree

And then define leaves, unary and binary operators as instances of Tree Float

[divarvel]

Good idea. I'll do that this evening. I'll try to make tree an instance of Foldable as well.