AST visitor should allow to transform the AST

[OlegIlyenko]

Similar to the reference implementation: https://github.com/graphql/graphql-js/blob/c23f5782dee81b0a59a6a6bb2bcca9da247570e7/src/language/visitor.js#L72-L94

[OlegIlyenko]

Here is an example of this feature. Here I just replace all variables with a constant:

import sangria.ast._
import sangria.visitor._

val query =
  gql"""
    query Foo($$aa: String = "some default", $$lang: String) {
      a: aaa(e: one)
      b: aaa(e: $$aa) {
        comments {
          # deeply nested
          name(lang: $$lang)
          ...Bar
        }
      }
    }

    fragment Bar on Comment {
      description(lang: $$lang)
    }
  """

val result =
  AstVisitor.visit(query, AstVisitor {
    case _: VariableDefinition ⇒
      VisitorCommand.Delete
    case _: VariableValue ⇒
      VisitorCommand.Transform(StringValue("some constant"))
  })

println(result.renderPretty)

result will look like this:

query Foo {
  a: aaa(e: one)
  b: aaa(e: "some constant") {
    comments {
      # deeply nested
      name(lang: "some constant")
      ...Bar
    }
  }
}

fragment Bar on Comment {
  description(lang: "some constant")
}

/cc @timsuchanek