canonicalize/normalize document utility

# Feature requests
Would a `canonicalize(node)` or `normalize(node)` utility be a desirable feature? I started [working on one](https://github.com/micimize/graphql-to-dart/blob/5de6b0e2cf51ce1a9a049d892a0f1dbfa50aafb2/src/canonicalize.ts) to simplify some aspects of ast-based code generation, though all it does right now is merge duplicate fields

I don't think it should inline fragments or anything like that (though maybe conditionally). Maybe we'd need to spec out exactly what a canonical document form would actually look like though