[pigeon] refactored the AST

[gaaclarke]

Over the course of working through backlogged PRs there was feedback on the structure of the AST. I've addressed them here by:

1. Introducing the concept of the TypeDeclaration
2. Renamed Field to NamedType
3. Transitioned the return value for methods to a TypeDeclaration

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[gaaclarke]

subsumed by #428