Union support

[andreas]

This PR adds union support to ppx_graphql. The major caveat is that you need to add the field __typename manually to your query for this to work. In the future, this will happen automagically.

Example:

query {
  search(query:"topic:mirageos", type:REPOSITORY, first: 5) {
    node {
      __typename
      ...on Repository {
        nameWithOwner
      }
    }
  }
}

[mato]

Thanks @andreas. I can't comment on the implementation, but have tested this and it works for me!