`lib.data` can make debugging extremely hard as fields are not written to VCD

This is related to the introduction of a `Display` statement. Ideally, we should be able to freely (as in, "for free", without having to post-process VCD files) use decoders that only use f-strings, with CXXRTL, since CXXRTL is about to learn how to format arbitrary data and then will presumably be able to put it into VCD files.

Verilator users are going to be in a pickle, however.