Add a first implementation of hooks

This is the first minimal implementation of how code generation hooks from downstream projects could look like.

There is a few more things to think about (feel invited to share ideas):

• How to document the arguments and return values expected from hooks
• How to handle multiple hooks registered to the same hook point and return values (this is quite relevant once you want to do loopy transformations in a hook. It means that you want to "chain" the hooks)

This fixes #129 (closed).