RFC: Client Controlled Nullability

[michaelstaib]

This PR implements the Client Controlled Nullability RFC.

graphql/graphql-spec#895

• AST
• AST Tests
• AST Docs
• Lexer
• Lexer Tests
• Lexer Docs
• Parser
• Parser Tests
• Parser Docs
• SyntaxPrinter
• Visitor
• Rewriter
• Operation Compiler
• Do we have changes in the execution or does a change in the operation compiler suffice?
• Benchmarks
  • Parser
  • OperationCompiler
  • QueryPlan
  • Execution
• Validation Rules
• Naming is not quite right yet