Add schema validation: Input Objects must not contain non-nullable circular references

[spawnia]

Input Objects are allowed to reference other Input Objects. A circular reference occurs
when an Input Object references itself either directly or through subordinated Input Objects.

Circular references are generally allowed, however they may not be defined as an
unbroken chain of Non-Null fields. Such Input Objects are invalid, because there
is no way to provide a legal value for them.

The following examples are allowed:

input Example {
  self: Example
  value: String
}

This is fine because a value for self may simply be omitted from the arguments.

input Example {
  self: [Example!]!
  value: String
}

This also works as self can just contain an empty list.

The following examples are invalid:

input Example {
  value: String
  self: Example!
}

input First {
  second: Second!
  value: String
}

input Second {
  first: First!
  value: String
}

Spec change: graphql/graphql-spec#445
Reference implementation: graphql/graphql-js#1359

I think this might be added to a 0.13 release, since the relevant spec change is just a clarification.

[vladar]

That's great, thanks!