Verify: some minor changes to the ref. impl.

[OlegIlyenko]

There were several minor changes is the spec. Verify whether there is anything that needs to be done in sangria:

• Fix unhandled error when parsing custom scalar literals. graphql/graphql-js#1126
• (Potentially Breaking) Allow serializing scalars as null. graphql/graphql-js#1104
• Add a utility to detect description changes graphql/graphql-js#1127 (already done via scehma comparator)
• Preserve original coercion errors, improve error quality. graphql/graphql-js#1133
• Better Predicates graphql/graphql-js#1137
• Find Breaking Changes: removed directives graphql/graphql-js#1146
• Improve dedent (fixes #1202) graphql/graphql-js#1203
• Execute serially supporting sync execution. graphql/graphql-js#1198
• Perf: memoize collectSubfields graphql/graphql-js#1130 (field collector already has a cache)
• Fix print of block string with leading space and quotation graphql/graphql-js#1190
• graphql/graphql-js@a722af7
• Allow interface type extensions graphql/graphql-js#1222
• Fix how TypeInfo handles inline fragments without type graphql/graphql-js#1041