[sgen] Add parallel copying infrastructure

[sgen] Add parallel copying infrastructure

Parallel copying should happen only when the world is stopped and the gc thread is not running. Each thread allocates from its own block freelist, which he obtains by pop-ing the global block free list.

When a worker has to copy an object, it first allocates a new object in a major block and initialises its contents. Afterwards it tries to forward the vtable, using a CAS operation. If the vtable already contains a forwarded object, we just return the forwarded address, and the object that we allocated will be left to die (we expect this case to be extremely unlikely, otherwise we could re-enque the free object in its block's freelist).

After the copying is done, the caller has the duty to update references and mark the new object (which we have to do it also with a CAS).

FIXME : This means that an object can reside on a gray queue without being marked, so another thread can enque again this same object. We should refactor this copying/mark/enqueing mechansim to avoid these issues.