Query visitor with schema type info

[vladinator1000]

🚧 Work in progress 🚧
Porting the graphql-js TypeInfo data structure to graphql-parser in order to get schema information as we walk the AST.

I've based this on @davidpdrsn's visitor work from #26 as discussed in my comment in the visitor trait issue.

⚠️ Currently stuck at figuring out how to implement it. Graphql-js has one enter and leave function visitor-wise, where @davidpdrsn's visitor has separate methods for each type of AST node.

Goals:

1. Get type info about the schema as you walk a query AST
2. Be able to mutate the AST as you see fit