Choose the Garbage Collector used by the translated program:

  - "ref": reference counting. Takes very long to translate and the result is
    slow.

  - "framework": our custom mark-and-sweep collector. Takes moderately long and
    is the fastest option without external dependencies.

  - "stacklessgc": same as "framework" but uses a different method to find the
    garbage collection roots on the stack, by unwinding it, using stackless:
    :config:`translation.stackless`.

  - "boehm": use the Boehm conservative GC


