Merge branch 'feature/vcl-traits-stuff' into 'master'

ref:extensions/dune-codegen The VCL does not export enough information about
its types to reason about them, when they are used as template parameters. In
the long run, a proper solution will be needed I guess\...

See merge request [extensions/dune-codegen!321]

  [extensions/dune-codegen!321]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/321