AST improvements

[rmosolgo]

I want to improve AST manipulation options in 1.9, including some breaking changes:

• Immutable AST nodes (breaking change)
  • replace attr_accessor -> attr_reader (Make language nodes immutable #1338)
  • freeze .children, .directives and other attached arrays (bc940ba)
• Add Node#path and Node#parent I decided against this because it doesn't support efficient modification, see Try to add path to AST nodes #1790
  • Since the nodes are immutable, we can cache these on the nodes themselves
  • [ ] Node#parent references the node where self is a member of children
  • [ ] Node#path is the static path to this node in the query string (see ... TypeStack, I think, where this is currently implemented)
  • [ ] consider removing parent from method-based visitor?
• Improve Language::Visitor
  • Build a method-based hook API ([Language::Visitor] use method-based visit handlers #1290)
  • Add manipulation options like GraphQL-JS (Manipulate AST with visitor #1740)
  • Deprecate .enter << usage (Add warning for proc-based visitors #2017)

Also, fix some things that this will break:

• (fixed) GraphQL::Client will be broken
• (logged) Make sure GraphQL::Libgraphqlparser will still work
• Other broken things to fix?