Now that the jextract declaration and Type APIs are more indepdendent from memory layouts, we should strive to move layout and function descriptor generation as close to code generation as possible. This is a necessary preparation step to start generating layouts and function descriptors strings directly from jextract types, which is a more robust way to do it (since it retains a lot of type information that is available in the jextract type API but that would be erased when moving to layouts).