Investigate performances of unification

[Drup]

There are cases where we have to fall down to proper unification. In this cases, it would be nice if it was fast. Ideally, the stat function would have an option to give us a "hall of fame" of the slowest queries. There are cases where it's expected, but I think we should collect examples that are really out of line.

• 'X * int -> 'X 'a * 'b * 'c * ('a -> 'd) * ('b -> 'e) * ('c -> 'f) -> ('d * 'e * 'f)