Support sum types in bidirectional conversion

Let's say, we have a type like this:

```haskell
data Either l r = Left l | Right r
```

It should be possible to write bidirectional converter for such type type. Perfectly with the help of `Alternative` type class. I think in TOML this should look like this:

```toml
foo.Left = 42
bar.Right = true
```

Specifying both constructors should result in parsing error.