Constructible / modifiable HIR

[goto-bus-stop]

Supersedes #590, and likely #580.

To unify the hir/encoder types, HIR types should be constructible. To modify documents (#420), the HIR is likely the nicest level to do it at (vs the low-level parser AST).

So the rough plan is:

• make the HIR nodes constructible (prob just add builders with buildstructor)
• have the whole HIR have one root node, so you can clone + modify a HIR
• add a serialization API to the HIR so you can use it instead of apollo-encoder
• add some basic modification methods for eg. adding fields or removing directives
• later integrate it with salsa possibly using a schema diff format??

[goto-bus-stop]

A pretty big issue i'm running into with having a single root node at the moment is that it more or less guarantees that there will be multiple Arcs pointing at the same definition nodes. That makes modifying them non-obvious, you can't just do Arc::get_mut() with multiple pointees and having a ton of mutexes in the hir seems quite bad/heavy weight.

[goto-bus-stop]

Our wording evolved a bit, so the issue is a bit outdated, as we are looking at having both a constructible HIR and a constructible "MIR". I'll keep it like this for now as we are still designing the higher level parts. Simon made a lot of progress on this branch: https://github.com/apollographql/apollo-rs/tree/mir

[SimonSapin]

I think we can consider this done, with the release of apollo-compiler 1.0 beta: https://docs.rs/apollo-compiler/1.0.0-beta.1/

[SimonSapin]

This was sort of a duplicate of #420