Make language nodes immutable

[tenderlove]

We have code in our client that "deep freezes" all AST nodes. I think we could eliminate that if we just made AST nodes immutable by default. I only found one place where the gem is mutating nodes, but I guess all those "writer" methods are public API, so this PR might only be good for a major version release.

I separated this PR to two commits because I think avoiding mutation internally has merit on its own.

[rmosolgo]

👍 Looks like a nice improvement to me, I want to check tomorrow to make sure it doesn't break graphql-client, but other than that, I think it's all clear!

[rmosolgo]

Somehow I thought I already merged this, but I didn't 😌

It's going to break graphql-client:

https://github.com/github/graphql-client/blob/master/lib/graphql/client/query_typename.rb#L21-L34

I like the idea of a more stable AST structure, but I'm going to change the target of this PR to 1.8-dev and merge it in a bit, when I have some time to update that code as well.

[tenderlove]

I'll look in to what it takes to make graphql-client not mutate nodes. We may need a more functional interface for manipulating ASTs.

[rmosolgo]

GraphQL-js has a really nice approach for this, I added a comment to the previous issue for improving AST manipulation: #455 (comment)

[rmosolgo]

There are still a few things to do before immutable nodes are really ready:

• consider freezing the arrays that make up.children, too, so it doesn't get changed underfoot (then .children could also be frozen)
• add Node#path (and maybe Node#parent?) to simplify working with the AST, and it can be quite efficient now that they're frozen
• improve Visitor (see [Language::Visitor] use method-based visit handlers #1290) to support manipulating the AST, so people have a new way of messing with hand-rolled ASTs

But this is a great first step, so merging into the dev branch!