Disallow the creation of arbitrary Form structs

[dodomorandi]

The capabilities of Form depend on the context in which it is used. At the current state there is no check to avoid invalid states. There are two main ways to solve the issue:

• Perform a runtime check on the Thing to validate the coherency
• Check the invariants at compile-time

It is not necessary to have only one of the two solutions, probably the best approach is to rely on compile-time checks when possible and perform all the remaining ones at runtime.