VM: big refactoring removing the gc_workhorse

VM: big refactoring removing the gc_workhorse

Each collector were a slot_visitor containing a gc_workhorse containing
a policy class. This commit removes the gc_workhorse and the policies
and instead "inlines" the common code in the collector bodies. So there
is more code duplication, but the control flow doesn't "jump around" so
much so it is easier to understand. It also makes the nursery gc faster
because its collection code can be optimized better without the
workhorse/policy system.