Manipulate AST with visitor

[rmosolgo]

It would be nice to have a way to programmaticaly modify GraphQL ASTs, but we don't have one yet. I propose a way to do it using the class-based visitor on the 1.9-dev branch.

Goals:

• Efficient
• Easy-to-use
• Respects immutability of nodes

So, this implementation will treat the AST as a persistent immutable data structure:

• By default, the tree is only traversed; no modification or duplication
• When a hook returns a new node, then:
  • the node's parent is shallow-copied; the copy is modified to include the new child
  • the copied parent is returned so that its parent may also be modified

TODO:

• Lots of nodes need reorganization to support persistent copying, for example:
  • Implementing .children_method_name (I sure hope that each node type has only one possible name!)
  • Refactor initialize_node to call set_attributes?
  • But, instance_variable_set is slow, consider another metaprogramming approach?
  • Maybe generate classes from configuration? There's a lot of duplication and I'm about to add a lot more.
• Implement more modifications:
  • modify node & replace child
  • delete node & remove child
  • create node & insert child
  • others?
• Add a guide

Fixes #455