AST node type documentation and refactoring

[raehik]

• Lots more Haddock documentation for unclear parts of the AST (e.g. individual constructors in large sum types).
• More named records where appropriate, intended for assisting generic derivations that use them (Aeson).
• Bug fixes and alterations to types. Mostly refactoring to better explain parts of the AST, but includes breaking changes.

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• [x] document important/breaking changes in changelog