Make structs and unions adhere to C++ standard

[gostefan]

Some structs and unions currently don't adhere to the C++ standard. They rely solely on the compiler implementation doing what we expect it to do.

This PR fixes the elements that don't adhere to the C++ standard.

Only the last to commits are actually part of this PR. All the others are from PR #99 but not basing this PR on that one would lead to conflicts when merging.