General conversion for custom HDL types

Regarding the support for custom HDL types, I got the question: "what is the format of the conversion info?". Currently, the answer now is clearly a dedicated converter, soon to be supported as plugins. However, it seems to make sense to be able to provide a general file format for this that the "compiler" can output. This thread is about discussing how such a format should look like.

The assumption is that there are "advanced" types that are converted to one or more bit-vectors (possibly other types) and that we would like to convert it back to the original type as good as possible. It may be that we would like this to happen not only as a translator, but also in the variable list (if a product type ("struct") is converted to several signals, this should show up in the variable list as a single signal).