Implement exists for Graphcool bindings

[kbrandwijk]

No description provided.

[schickling]

Would be great to have this soon! 🎉

[kbrandwijk]

Do we have final consensus on the API?

1. context.db.Post.exists(...)
2. context.db.posts.exists(...)
3. context.exists.Post(...)
4. context.exists.posts(...)

(1) is my personal favorite (by Type name, not query root field).

[schickling]

For now it will be (4) until we've found a new Binding API design (see prisma-labs/prisma-binding#11)

[kbrandwijk]

After some consideration, the current PR is for option (3), with a corresponding PR on graphcool-binding to implement identical behavior for dynamic bindings. See prisma-labs/prisma-binding#34.