We should cut a spec release

[benjie]

Let's set a deadline of July; anything not merged by then can wait for the next release. This gives us 2 months to get the spec release finalized before GraphQLConf.

We've had formatting, minor editorial (including definitions), and typo fixes, but also:

• Allow deprecation of input values (field args, directive args, input fields) graphql-spec#805
• RFC: Allow full unicode range graphql-spec#849
• Make the reason argument in @deprecated non-nullable graphql-spec#1040
• Add explanation about argument name uniqueness. graphql-spec#891
• Be strict about error paths format graphql-spec#1073
• ID must always serialize as String would graphql-spec#1086
• Fix coercion table for list graphql-spec#1057
• Fix ambiguity around when schema definition may be omitted graphql-spec#987
• Fix reference mistake in subscription execution graphql-spec#994
• Clarify that selection sets cannot be empty graphql-spec#1025
• Add links to contributed custom scalar specs at scalars.graphql.org graphql-spec#1009
• Add explicit definition for BlockString graphql-spec#1042
• Add definition of "selection set" and clarify serial execution examples graphql-spec#1032
• Reformat the parts of an execution request graphql-spec#1090
• Change 'original' to 'previous' to clarify multiple extensions graphql-spec#1123
• Rename VariableDefinitions to VariablesDefinition graphql-spec#916
• Leaf field selections clarification graphql-spec#958
• Editorial: Remove inaccurate statement about line terminator within tokens graphql-spec#963
• Editorial: Fix reference to object in interface introspection graphql-spec#964
• Editorial: Clarity about subject being a GraphQL service or system graphql-spec#965
• Editorial: Error Terminology graphql-spec#966
• separate out IsSubType from IsValidImplementationFieldType graphql-spec#977
• P34: implementing field type is either exact match or of covariant type graphql-spec#974
• P30: Fixed the explanatory text for algorithm checking uniqueness of non-repeatable directives graphql-spec#975
• field merging - field TYPES must not differ graphql-spec#979
• Fix 'response error' -> 'request error' graphql-spec#1016
• enhance(ResolveFieldValue): note that list items may be async graphql-spec#1066
• Editorial changes for Event Streams graphql-spec#1099
• Editorial: data and errors appear in the "response" graphql-spec#1130
• Editorial: move "Path" to it's own section graphql-spec#1129

We've got some really big topics coming up, including fragment arguments, nullability, and incremental delivery; let's get the above list of features out as the latest stable version of the GraphQL spec so that these new major topics can start from a (relatively) clean slate.

I think we should aim to cut a new spec release roughly every 18 months, and we're long overdue (at 40 months).