[Tracking issue] going farther than the AST

[scrabsha]

Ideally we should have our own types, that are a bit more abstract than what syn currently provides. For instance, we don't need to keep the fn token when storing a function signature. Similarly, we don't need the :: when storing an item path.

One of the greatest benefits is that we could do some simple operations during the conversion. A simple example is trait bounds. cargo-breaking currently treats the change T: Send + Sync to T: Sync + Send as a breaking change. As a fix, we could alphabetically sort the bounds when converting data to our own types.

This issue lists what needs to be implemented to fit these goals:

• Simplify internal type representation #4,
• Simplify internal function signature representation #5,
• representation of trait items (consts, methods and types),
• reworking import representation, if needed.

This list is not exhaustive, some items may be added in the future.

[scrabsha]

Given the progress of the rustc backend (see #19), this issue is likely to get obsoleted soon.