Add a records field to the root query

[haydn]

It's often useful to query for a record, a set of records or all records without specifying the type. There should be root-level fields that use the generic Record (currently Node) interface.

These are the default fields that should exist on the root query (this assumes #3 has been implemented):

type RootQueryType {
  relationships(foreignKeys: [ForeignKeyInput!]): [Relationship!]!
  record(id: String!): Record
  records(ids: [String!]): [Record]!
}