Monoid library

Hello, For the purposes of my monad library I wrote a small monoid library that I am attaching to this mail. Instead of using overloading, it makes the monoid structure explicit. In this way we can have both additive and multiplicative monoids on the same type, and for my library it removes the 'Monoid' constraints that were all over the place (the same happens in 'mtl'). The version that I post here coexists with the overloaded approach, but I am not sure that both of them are needed. If people think that this way of doing monoids is useful, perhaps we should update the Data.Monoid. -Iavor