Effectively deep-freeze Query objects

[0xMochan]

Is your feature request related to a problem? Please describe.
subgrounds/query.py has all objects related to the AST of GraphQL. Currently, every object is frozen, but nested fields of said objects are not (such as nested lists and dicts). This means the greater objects of Document, etc are not hashable.

Describe the solution you'd like
Ensure all query objects are hashable by using immutable data structures.

Additional context
Likely should be considered with a general refactor / subpackage breakdown of subgrounds/query.py).

Implementation checklist

• Task 1