similar to what primitive-fork in guile:

1. scm_i_finalizer_pre_fork () == scm_set_automatic_finalization_enabled (0)
2. stop_signal_delivery_thread ()
3. disable-gc
4. assert length (threads) == 1
5. run exp
6. enable-gc
7. scm_set_automatic_finalization_enabled (1)
8. scm_i_signals_post_fork () or scm_i_ensure_signal_delivery_thread

Problem: none of the calls in 5 are part of the API (SCM_INTERNAL)