Extend -print-transformations

[Kakadu]

I'm currently developing a deriving rewriter which is extending type-conv and is being registered using Type_conv.add "name". When I run -print-transformations the standalone rewriter gives me only type_conv. It would be great if it will print all the generators registered for type_conv.

P.S. I actually have a bug at the moment that standalone rewriter is not linked with my code and complains like Error: ppxlib_type_conv: 'name' is not a supported type type-conv generator. It would be great to know for sure (using switch above) that the library that registers generator is not linked.

[ghost]

Yes, that makes sense. Currently type_conv is seen as a single rewriter by the rest of the machinery, so it's not straightforward to support this feature. It might be easier after #14.

For the bug, could you open a separate issue with a bit more information about how to reproduce it?