Track draft specification

[lirsacc]

A few RFCs have been accepted and should end up in the next version of the spec. This issue is here to keep track the ones we've identified. See accepted RFCs and draft RFCs for the full list.

Non breaking changes should be implemented directly while the rest could be hidden behind a flag.

• Clarify that lexing is greedy graphql/graphql-spec#599: 3f367d5
• RFC: Number value literal lookahead restrictions graphql/graphql-spec#601: 3f367d5
• [RFC] Allow directives on variable definitions graphql/graphql-spec#510: 8b4f7e5
• [RFC] Repeatable directives graphql/graphql-spec#472 (5e910bc)
• [RFC] "Directive order is significant" section graphql/graphql-spec#470: This is already the case for schema directives, executed in the order they appear in the AST and it is up to consumers to handle this for runtime directives, this may require some changes when looking at repeatable directives
• RFC: Allow interfaces to implement other interfaces graphql/graphql-spec#373
• [RFC] Custom Scalar Specification URLs graphql/graphql-spec#649
• Add description to Schema graphql/graphql-spec#466 (Schema description #7)
• Disallow non-breakable chains of circular references in Input Objects graphql/graphql-spec#445 (bc706f3)