Add the `flap` function to `Apply` to facilitate combining a function in an
`Apply` context with a value which is not.

This is similar to `ap`:

```
def ap[A, B](ff: F[A => B])(fa: F[A]): F[B]
```

with the minor change of the value not being in a context:

```
def flap[A, B](ff: F[A => B])(a: A): F[B]
                              ^^^^
```