fn: reimplement internals of either, add tests

fn/option.go

@@ -218,30 +218,18 @@ func (o Option[A]) UnsafeFromSome() A {
 // providing the Right value that should be used if the Option value is None.
 func OptionToLeft[O, L, R any](o Option[O], r R) Either[O, R] {
 	if o.IsSome() {
-		return Either[O, R]{
-			left:  o,
-			right: None[R](),
-		}
+		return NewLeft[O, R](o.some)
 	}
 
-	return Either[O, R]{
-		left:  None[O](),
-		right: Some(r),
-	}
+	return NewRight[O, R](r)
 }
 
 // OptionToRight can be used to convert an Option value into an Either, by
 // providing the Left value that should be used if the Option value is None.
 func OptionToRight[O, L, R any](o Option[O], l L) Either[L, O] {
 	if o.IsSome() {
-		return Either[L, O]{
-			left:  None[L](),
-			right: o,
-		}
+		return NewRight[L, O](o.some)
 	}
 
-	return Either[L, O]{
-		left:  Some(l),
-		right: None[O](),
-	}
+	return NewLeft[L, O](l)
 }