GraphQL 1.9 support

[rmosolgo]

GraphQL 1.9 has immutable AST nodes, which breaks this library, because this library's basic flow is to instantiate a node, then assign values to it during the visit.

I think the library could be refactored to initialize a Hash, accumulate values in the hash, then instantiate a node when all the attributes are present.

[relwell]

@rmosolgo I think the README for this library merits a deprecation warning. I spent a couple hours this morning digging into this exact problem. If your recommendations in rmosolgo/graphql-ruby#2398 were mentioned there, I think it'd save a lot of time for those interested in implementing this functionality.