ast-exporter: Export some macro arg expansions

[Rua]

• Depends on ast-exporter: In VisitExpr, handle when Begin and End start off in different macros #1795

The "chain" of expansions is a little different for macros with args (that is, functional macros). This PR does two things:

• Handles arg expansions differently, so that const macros that call functional macros are now expanded.
• Exports the arg info to the Rust side to be used to hopefully make functional macros possible later.

Not all macro args are currently handled by this because of technical limitations. This code is only able to "see" macro args in the macro where they were expanded into tokens. They are not visible in macros that take an arg and then pass it on to another macro call in an arg. Maybe a solution will eventually be found for that. For now at least, the AST on the Rust side contains expressions that are macro arg expansions!