Add support for using a CodeBlock as a value to generate

[snicoll]

When transforming data in custom code, it would be beneficial to be able to reuse the built-in converters while being able to take control over an element of the chain.

An example is Map<String,T> where T is written as a local variable. Once that's done, it should be possible to let the generator write the code for Map<String,TheNameOfTheField> but such indirection isn't possible at the moment.

Checking if the value is a CodeBlock and then render that as is should help as code generation doesn't have to comply with the actual type of what the generated code represents.

[snicoll]

Reviewing this in detail, there's an easy way to break things, especially for collections as they tend to be generated in separate methods that don't carry the context. For cases like this, it's better to create an empty Map and loop over them to add the entries. This also avoids problem of having to assign a field name with something that may not be a valid identifier.