(Fix) Add support for mix mode queries.

[paulpdaniels]

This PR adds support for mixed mode queries, i.e. queries where there are other non-rest query parts that need to be forwarded to other links. It should support both sibling queries as well as nested queries. Though based on my tests there are some issues (which may not be related to apollo-link-rest) with some advanced use cases e.g. sending an export variable to a second generation nested field across a link boundary,

query {
  authors {
    id @export(as: "authorId")
    recentlyViewedPosts @client {
      id @export(as: "postId")
      # Doesn't seem to resolve the authorId in this case
      post @rest(type: "Post", path: "/posts/{exportVariables.authorId}/{exportVariables.postId}") {
        body
      }
    }
  }
}

@fbartho @ivank

Tests expanded from #137

Fixes #97

[paulpdaniels]

@fbartho any idea why the tests pass, but the compile fails?

[fbartho]

@paulpdaniels Some mess around rollup / npm build process causes some duplicate shit in a nested node_modules, I forget what the exact cause is, but I've found it's nothing to worry about, and I can resolve it when building to deploy.

Sorry it took me so long to get back to this PR. I'm out of crunch, and back from vacation, and landing this now, and deploying these changes soon! Thanks for your contributions.

I'd love to get a fix for that issue eventually, but I think we might need @hwillson to give us a hand.