Merge branch 'feature/code-generation-hooks' into 'master'

ref:dominic/dune-perftool 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):

-   \[x\] How to document the arguments and return values expected from hooks
-   \[x\] 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].

See merge request [dominic/dune-perftool!277]

  [#129]: gitlab.dune-project.org/NoneNone/issues/129
  [dominic/dune-perftool!277]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/277


Closes #129