[Meta] Using the Rustc API by default

[scrabsha]

Implementation on the main branch parses an AST in order to get breaking changes. The rustc-backend branch uses the Rustc internal API, which makes changes easier to spot and way more precise. In order to use the Rustc API by default, we must implement look for changes in the following items:

• functions (we currently only track when the number of arguments change) (implementation, tests),
• methods (implementation, tests),
• trait definition (implementation, tests),
• trait implementation (implementation, tests),
• struct and enum (implementation, tests (struct), tests (enum)).

Making all the linked integration tests pass is required in order to merge rustc-backend to main.