Expose toEffect and fromEffect (#418)

Expose `toEffect` and `fromEffect` as public API.

Use case: with `doobie.ConnectionIO` to use `zio` effects in transaction with queries.
```scala
for
    r1 <- selectSomethingForUpdate
    _ <- log.info(s"selected: $r1").toEffect[ConnectionIO]
    r2 <- updateSomething(r1)
yield r2
```

> I agree that and `fromEffect` seem generically useful. 
(c) @adamgfraser

zio-interop-cats/shared/src/main/scala/zio/interop/package.scala

@@ -72,22 +72,22 @@ package object interop {
         }
     }
 
-  @inline private[zio] def fromEffect[F[_], A](fa: F[A])(implicit F: Dispatcher[F]): Task[A] =
+  @inline def fromEffect[F[_], A](fa: F[A])(implicit F: Dispatcher[F]): Task[A] =
     ZIO
       .effectTotal(F.unsafeToFutureCancelable(fa))
       .flatMap { case (future, cancel) =>
         ZIO.fromFuture(_ => future).onInterrupt(ZIO.fromFuture(_ => cancel()).orDie).interruptible
       }
       .uninterruptible
 
-  @inline private[zio] def toEffect[F[_], R, A](rio: RIO[R, A])(implicit R: Runtime[R], F: Async[F]): F[A] =
+  @inline def toEffect[F[_], R, A](rio: RIO[R, A])(implicit R: Runtime[R], F: Async[F]): F[A] =
     F.uncancelable { poll =>
       F.delay(R.unsafeRunToFuture(rio)).flatMap { future =>
         poll(F.onCancel(F.fromFuture(F.pure[Future[A]](future)), F.fromFuture(F.delay(future.cancel())).void))
       }
     }
 
-  private[zio] implicit class ToEffectSyntax[R, A](private val rio: RIO[R, A]) extends AnyVal {
+  implicit class ToEffectSyntax[R, A](private val rio: RIO[R, A]) extends AnyVal {
     @inline def toEffect[F[_]: Async](implicit R: Runtime[R]): F[A] = interop.toEffect(rio)
   }
 }