Add option to pass expanded output to rustc

[bluebear94]

This would make it easier to debug compilation errors inside the expanded code, since such errors highlight the code in the use of the macro instead of the expanded code.

[dtolnay]

The expansion process is not designed to necessarily create compilable source code, so I would prefer not to add this. It's only a debugging aid for human inspection.

[bluebear94]

Thanks for the answer. The reason I asked is that the expanded code can be large, and figuring out where a typing error in the expanded code is pointing to can be helpful.