Implements the algorithm for validating "Field Selection Merging" as described in:
https://tech.xing.com/graphql-overlapping-fields-can-be-merged-fast-ea6e92e0a01
Should have the same effect as the algorithm in the GraphQL Specification:
https://graphql.github.io/graphql-spec/draft/#sec-Field-Selection-Merging
Implements the algorithm for validating "Field Selection Merging" as described in: https://tech.xing.com/graphql-overlapping-fields-can-be-merged-fast-ea6e92e0a01
Should have the same effect as the algorithm in the GraphQL Specification: https://graphql.github.io/graphql-spec/draft/#sec-Field-Selection-Merging