Tweak signature of {set,modify}F

Follow up to #97. Without this tweak, Scalac complains during function composition.

Example:
```scala
a.setF(3) compose b.setF("hi")
```
Oh Scala...
```scala
val setF1[S, A, B](f: A => B)(from: S) = ??? // before this patch
val setF2[S, A, B](f: A => B) = (from: S) => ??? // after this patch

def example1[S](f: S => S) = ???
example1(setF1(???)) // ← compiles
example1(setF2(???)) // ← compiles

val example2 = setF1(???) compose setF1(???) // ← doesn't compile, needs _
val example2 = setF2(???) compose setF2(???) // ← compiles
```