Visitor Fixes and Editing Support

[NeedleInAJayStack]

This adds editing support to the AST Visitor system. In addition, it also:

• Adds all graphql-js visitor tests cases, both for editing and for normal visitor operation
• Fixes a bug where the Visitor would not visit all Directive, ListType, or ListValue properties
• Fixes a bug where Visitor path would be generated incorrectly
• Improves Visitor null safety

[NeedleInAJayStack]

@paulofaria Looks like the any keyword isn't supported in Swift 5.5. What do you think about dropping support for that version?

[paulofaria]

Hm, I'm not sure we can, based on the SSWG guidelines? Can't we just remove the keyword so it's backwards compatible?

[NeedleInAJayStack]

Oh weird - I added any because I thought I saw error messages, but I wasn't able to recreate them. I've removed the any keywords.

Maybe not relevant anymore, but I don't think SSWG requires extensive backsupport - New NIO versions only back-support to Swift 5.7: https://github.com/apple/swift-nio/#swiftnio-2

[paulofaria]

Beautiful!

[NeedleInAJayStack]

Thanks @paulofaria. I just made one last small change to back-support the public Node Protocol API, making this a minor change instead of major.

Mind approving again?