Simplify syntax diagrams

Coming back to the off-topic point in #1798: Some of the syntax diagrams can be simplified quite a bit by removing superfluous elements. There are basically two ways to solve this:

• Implement an optimizer that takes the grammar's AST and performs the simplification automatically. Such as here.
• Manually take care not to introduce superfluous elements in the grammar rules themselves. There is a conflict of goals here, as the rules become more compact/nested. That is, the visual representation becomes easier to follow at the cost of the textual representation becoming harder to understand.

See #1798 on what this does to the syntax. Do you fancy a PR for the either of those options? The second one is obviously much more simple to do.

Start by reading the context in #1798 and comparing the six attached syntax diagrams with their grammar rules. Review the two proposed approaches, including the optimizer example in lowering.rs, then determine which scope is appropriate; done means the superfluous diagram elements are removed without changing the documented syntax.