…eplaced with Java8 input types where possible

 - Fn1 argument types become j.u.f.Function
 - Fn2 argument types become j.u.f.BiFunction
 - Adding Fn2#toBiFunction

 - Well, Java8's type hierarchy and javac really worked together on this one. Not only is it not possible for both Fn1 and Fn2 to extend their
    native Java8 counterparts (j.u.f.Function and j.u.f.BiFunction, respectively), due to Function and BiFunction existing in orthogonal type
    hierarchies and both defining conflicting methods with the other; it is also not possible to provide overloads for both Fn1 and Function
    (or Fn2 and BiFunction), since both will be @FunctionInterfaces, and javac will have no opinion about precedence of one over the other.
    The outcome is that explicit, dreadful type-casts to something like (Fn1<Something, OtherThing>) or
    (BiFunction<First, Second, Output>) would be necessary at the call site of every overload. This is the least acceptable outcome for me,
    so I'm begrudgingly just typing against native Java8 types for all inputs. I'm sorry. This is why we can't have nice things.