Add `fromLeftM` and `fromRightM`

I often find these functions helpful and end up defining them:

```
fromLeftM :: Monad m => (b -> m a) -> m (Either a b) -> m a
fromLeftM = eitherM pure

fromRightM :: Monad m => (a -> m b) -> m (Either a b) -> m b
fromRightM f = eitherM f pure
```

I was wondering what you thought about including them in `extra`?