XRBK macro: restructure around a mode system

[Antikyth]

XRBK macro should be able to be run in two modes: one for normal expansion, and the othe for expansion specifically for IntelliJ: the second mode would have code generated which makes IntelliJ have a better understanding of how to provide suggestions.

For example, attributes wouldn't be removed (e.g. derives), which would mean IntelliJ can present them as being actual attributes, when in reality they are still removed as far as the compiler is concerned. This is recommended by IntelliJ - they set an environment variable during macro expansion specifically to allow for this functionality.

This probably requires some restructuring of XRBK macro. While doing that restructuring, it would be good to make it easier to define new custom attributes in XRBK macro - just something to think about. Also, the code for sources is very difficult, perhaps that can be improved somehow.