Helpers around the different key formats used in the resolver, builder and validator states to index sub schemas, referenced schemas, anchor or meta schemas.

For instance:

• We have a {"$ref": "http://some-schema/#$defs/order"} JSON schema.
• The builder will build the $ref keyword as a key: {:pointer, "http://some-schema/", ["$defs","order"]}.
• The builder, via the resolver, will fetch http://some-schema/, store it locally and build validators. Those validators will be stored under the same key ({:pointer, "http://some-schema/", ["$defs","order"]}) in the root schema.
• When the validator will validate the reference, it will fetch that key from the root schema and apply the retrieved validators to the data.