Restructure graphql modules

[sandreae]

Moved modules and methods around in graphql so they better reflect the design patterns we're following. Now things look like:

├── constants.rs
├── mod.rs
├── mutations
│   ├── mod.rs
│   └── publish.rs
├── queries
│   ├── all_documents.rs
│   ├── document.rs
│   ├── mod.rs
│   └── next_args.rs
├── scalars
│   ├── document_id_scalar.rs
│   ├── document_view_id_scalar.rs
│   ├── encoded_entry_scalar.rs
│   ├── encoded_operation_scalar.rs
│   ├── entry_hash_scalar.rs
│   ├── log_id_scalar.rs
│   ├── mod.rs
│   ├── public_key_scalar.rs
│   └── seq_num_scalar.rs
├── schema.rs
├── tests.rs
├── types
│   ├── document_fields.rs
│   ├── document_meta.rs
│   ├── document.rs
│   ├── mod.rs
│   └── next_arguments.rs
└── utils.rs

Also moved dynamic schema build and resolve methods onto "empty" structs representing graphql types cos it makes the api quite nice.

Naming is still not quite there, especially whether the types should be called document_x or schema_x, but I will tackle that incrementally over the next few PRs.

📋 Checklist

• Add tests that cover your changes
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