implement higher-order unification for type constructor inference

implement what's described in "Partial polymorphic type inference and higher-order unification"

```scala
object Test {
  def meh[M[_], A](x: M[A]): M[A] = x
  meh{(x: Int) => x} // should solve ?M = [X] X => X and ?A = Int ... 
}
```