Handle Changeable/Buildable child fields (nested classes)

[Nico04]

When you have a nested class structure (like in your basic_example, with Product beeing a child field of Order), I'm a bit surprised that generated code for changeable and buildable doesn't handle that :

• _OrderChanges.product is of type Product and not _ProductChanges
• OrderBuilder.product is of type Product and not ProductOrder (however this makes more sense as Product is NOT buildable)

I have a lot of nested classes like that, so this limitation makes the whole generated code a lot less usefull unfortunately.
Maybe we can add an extra parameter for this ?

[BreX900]

It would require more generated code and the performance would decrease both for the generator and the running code. Let's say that I almost never clone/change/rebuild a class. Do you have this need very often?

I'm also waiting for the metadata to arrive in stable so I can retire the build_runner. Sorry but I don't think I have any plans to support this