Squashed 'native/sylvan/' changes from ac97e16..c0e9bf5

c0e9bf5 Bugfix: garbage collection did not properly mark BDDs that were the result of stolen and completed tasks. Now, every spawned Sylvan task is stored in a thread-local array. During garbage collection, the results of completed tasks are now also marked as in use.

git-subtree-dir: native/sylvan
git-subtree-split: c0e9bf5edcd8a0156ae566e2f8aa6f9d91a67d30
Update Sylvan to latest version