Struct support - vspec2franca

This issues concerns struct support for vspec2franca.

Required work:

- Define/propose how it shall be handled (possibly in this PR)
- Implement and test
- Make sure that expected output format is described in documentation
- If feasible, add at least one test case checking output

_VSS Struct support is expected to be officially released for VSS 4.0, late April. To identify and manage remaining a work number of issues have been created to facilitate more detailed discussion and to identify volunteers for implementing/reviewing/testing._

**Volunteeers wanted - please comment if you are willing to help on vspec2franca!**