The `FlatMap` type-class includes a function `ifM`, which allows
evaluating a Boolean computation and, depending on the result,
choosing between one effectful computation or other.

We add to the `Apply` type-class a very similar function.

Although the function is similar to `ifM`, there is an important
difference between them, as to how they handle the effect:
- The `ifM` will run the effects of the condition `F[Boolean]`,
  followed by the effects of _only one_ of the branches.
- The `ifA` will always run the effect of the condition, that of the first
  branch, and that of the second branch, in that order.

Thus, the if-then-else part in `ifA` only applies to the inner values,
not to the effects.

To preserve binary compatibility, we can not add methods to
an existing type-class, and can only use the syntax extension.