Choose method how to find roots in the GC. Boehm and refcounting have their own
methods, this is mostly only interesting for framework GCs. For those you have
a choice of various alternatives:

 - use a shadow stack (XXX link to paper), e.g. explicitely maintaining a stack
   of roots

 - use stackless to find roots by unwinding the stack.  Requires
   :config:`translation.stackless`.  Note that this turned out to
   be slower than just using a shadow stack.

 - use GCC and i386 specific assembler hackery to find the roots on the stack.
   This is fastest but platform specific.

 - Use LLVM's GC facilities to find the roots.