Take into account vc:do_not_keep_trace in let-rec definitions.

There was a FIXME about this in vc.ml. This MR fixes it.

There is one thing I am not absolutely confident about my implementation: the new implementation uses a new environment for every definition of the same group of mutually recursive functions, while the old implementation used the same environment. This matters because an environment contains a gensym for exception identifiers. Hence, with the new implementation, functions in the same group can alias exception identifiers, while they were unique in one group in the old implementation. I don't think this matters, but my understanding of this piece of code is too low to be fully confident.