Validation rule changes

[OlegIlyenko]

• Loosen overlapping field validation rules [validation] Loosen overlapping field validation rules graphql/graphql-spec#120
• Allow differing directives.[validation] Allow differing directives. graphql/graphql-js#230
• perf improvements for fragment cycle detection [Validation] perf improvements for fragment cycle detection graphql/graphql-js#231
• Factor out and memoize recursively referenced fragments [Validation] Factor out and memoize recursively referenced fragments. graphql/graphql-js#234
• Memoize collecting variable usage. [Validation] Memoize collecting variable usage. graphql/graphql-js#235
• Allow safe divergence graphql/graphql-js@c9c56f2
• Remove visitFragmentSpreads [Validation] Remove visitFragmentSpreads graphql/graphql-js#237
• Parallelize validation rules [Validation] Parallelize validation rules. graphql/graphql-js#238