Builder type should not extend model object

[SimonCockx]

Right now, generated builder classes extend the model object they are supposed to build, which is error-prone.

Example: checking equality between two model objects might return False although they are exactly the same, just because one of the arguments is a builder. This leads to hard-to-spot bugs in the generated code, which might have been spotted early on by a static compilation issue if the builder class wouldn't extend the model object.

[SimonCockx]

Related: #632, #633

We should think carefully about the exact use cases we want to support.