Add `asumMap`

There're useful `asum` function:

```haskell
asum :: (Foldable t, Alternative f) => t (f a) -> f a
```

But sometimes it's desired to have following function:

```haskell
asumMap :: (Foldable t, Alternative f) => (a -> f b) -> t a -> f b
```

(not sure about the constraints)