Unify execution metadata

[rmosolgo]

There are a lot of non-user-facing objects created to resolve a GraphQL query:

• Tokens
• AST nodes
• Irep nodes
• FieldResolutionContexts
• FieldResults
• SelectionResults
• (and plenty more: Lazies, LazyConnectionWrapper, Irep Scopes ... 😬 )

I think some of these could be merged into one: FieldResolutionContext, FieldResult, SelectionResult. They all form a tree which corresponds to the final result. Merging them would open some interesting doors:

• Traversing the tree of objects & results at runtime (eg How can I get the parent fields from ctx? #881)
• Deferred/streaming execution (if nodes retain their runtime objects and know how to continue executing themselves, a bit like ExecFrame from Defer directive #168)

Also, it would be a performance improvement due to fewer allocations (#861)

I don't have a priority for this; I want to get a few features finished out first. But I thought I'd open the issue so people know that the idea is out there!

[rmosolgo]

Starting in #923

[rmosolgo]

This is finsihed on the 1.7.x branch