Make independent of jlrs

[Taaitaaiger]

While the main goal of this package is generating wrappers that can be used by crates that use jlrs, this is an annoying limitation for people who want to use this functionality without depending on jlrs (e.g. a library that will be distributed as a JLL that doesn't depend on jlrs but does need the layout of some struct). In order to better serve that use-case I want to be able to generate wrappers that don't derive any jlrs-specific traits and make no use of the wrapper types defined by jlrs.